.25 Marauder Bullpup Build
This is a quick concept drawing of my .25cal M Rod before and after my many hours of labour…
The inspiration of the stock design is based around the Russian Hooligun Bullpup (The model before the Cricket)
The Mods Will include:
*New Bullpup walnut stock
*Shortened barrel (3″)
*New larger diameter shroud with Improved baffles and suppression
*New trigger and linkage
*Forward scope mount
*Single shot tray
This Is not the kind of job I would consider taking on for someone else..
This thread is more of a documentation and a “how to” guide for those capable or have the funding to pay a machinist copious amounts to make some of the parts for them so they can assemble.
I must add this is not the usual way I would go about this project either.
I started with a single cut and paste picture on photoshop and drew up nothing but the scope mounts. Everything was measured as I went and evolved along the way.
This is where the artistic licence came in as this is a one off piece of “art work”
All of this including the stock could have been designed on Cad and converted to DXF file leaving only the go button to be pushed on the CNC tooling I have at my disposal.
This method could be done in under half the time it took me to do it all by hand to a higher precision std but that would make me a pretty crappy artist now wouldn’t it?
The piccatinny riser mount was the most expensive part at $145, it is made by Steyr.
I could make one from a $40 blank piccatinny rail bar and machine it to the right shape but I chose not too as I already had the Steyr part.
Walnut slab: approx $35 (only used about a third of it)
Recoil pad: $30
Sandpaper and flapper wheels etc: $25
Cap screws grub screws and a selection of other hardware $12
New seals throughout, (bought 3 spares of everything): $14
I have roughed out a plan to make the scope mount and forward trigger group unit.. consists of a 30mm block of 6061 T6 alloy… some serious milling needed but you get the idea Im sure..
The pressure gauge will be nicely recessed up under the stock in the thumb hole cut out area.

The BSA Lonestar carbine barrel fitted to this rifle is already a few inches shorter than the original made by Green Mountain
barrels so I wont have to shorten it any further as there is plenty of room for the new suppressor baffles.

My first little job was to tidy up the rear barrel shroud thread
The new BSA Lonestar barrel has a little more girth than the original Green mountain barrel so it was turned down to 1/2″ to take the rear suppressor locating bush.
The bush also acted as a depth stopper when fitting the barrel so something needed to take its place when I removed it. This left an
ugly step in the barrel so I have turned down a press fit blind fit spacer tube to the same diameter as the barrel.. and set up the correct length
for the depth stop. With a little bit of bluing it looks like it was never there..

Next thing to tackle was the new single shot adaptor to manually feed those giant pellets.
Took some precision measurements for this little piece, I didnt want any feeding or rubbing issues.
I put a bit of 6061 T6 alloy into my mill and attacked it with various bits… the outcome was pleasing.
With a bit of matt black anodizing along with the rest if the new shiny parts it will do the trick.

Here it is all fitted up. Feeds perfectly!
Also took the opportunity to replace the transfer port seals between the main valve and the barrel port while I have the barrel off. The ones I removed looked a “wee bit” suspect.

I made a start on the scope and forward trigger group mount tonight.. that T6 alloy is hard stuff and slow going for my little machine..

I have stripped the trigger assembly and removed the safety catch, no need for it and it gets in the way of the new trigger connecting rod. I am still undecided whether to attach the new “pusher” connecting rod to the existing trigger like most mods I’ve seen or remove the trigger lever and make a “reverse pull” one with the con rod entering through a new hole in the front of the trigger group. I think for preserving this rifle so it can be returned to its std stock if so desired the first option will be less intrusive but the second option will make for easier stock in-letting and probably a better feeling more adjustable trigger.. I will sleep on that one..

I decided not to modify the rifle in any major way that wasn’t reversible so I’ve decided on a pusher extension linkage with adjustable length.
I made a pivot point block to slip on the trigger held in place with a grub screw. The length is adjustable on the stainless steel rod with locking nuts at both ends.
The remote trigger is 99% finished and I’m very happy with the way it functions. It is very adjustable and I’m sure I will be able to fine tune it to a very desirable trigger.

The trigger will be mounted to a block attached between the scope mounts like this..

I had a quick play on the lathe turned up the one piece baffle system for the new larger diameter suppressor…
The outer tube will slide over the barrel with the rear locating bush/seal (grub screwed to the barrel like the original)
and this baffle unit will screw on and lock it all into place. It is a very light weight suppressor weighing just under 100 grams!
The air flow test was impressive so I look forward to test firing this one..
The bottom pic is of the the new larger volume suppressor tubing next to the factory shroud. Its hard to tell from the pic but the new
tube is a larger diameter, the old shroud actually fits neatly inside the new tubing.


The new suppressor is finished apart from a couple of rubber ‘O’ rings and some grub screws to locate the rear bush.
I turned down the main body tube to just clear the air cylinder when fitted so this barrel remains totally free floated.
Weighs 95 grams in total. The original factory suppressor weighed 185 grams.
Another bunch of shiny new parts destined for the electro platers š


As I will no longer be using the factory barrel band I turned down a new spacer for the front of the cylinder out of a piece of 1 1/4″ alloy solid bar.
It has tidied that area up nicely.

I have the scope mounts polished inside the cylinder holes now and they are slipping on nicely so I can now finish work on the trigger, then its on to the time consuming dusty part.. the walnut stock…


With most of the engineering part finished I’ve turned my attention to the stock.
I spent about an hour moving the template around the slab trying to envision the finished stock… happy with the piece but wont really see the true grain until I start the shaping. I should have taken a pic of the vast array of router guides and jigging I had set up to get the inletting right, It looked awkward to put it best.. This is my first stock Ive made by hand that is not to be a painted finish so Ive been sooooo careful not to slip with any power tools..
that said there’s a lot of work to do on it yet so if you see a sudden shift from the idea of polished walnut to a nice olive drab paint job, you can assume Ive muffed it up and had to bog something :rolleyes:
<a href=”http://www.flickr.com/photos/87230484@N02/7987930813/” title=”IMG_0985 (Small) by wingman177, on Flickr”><img src=”http://farm9.staticflickr.com/8314/7987930813_ca67b4e285.jpg” width=”319″ height=”480″ alt=”IMG_0985 (Small)”></a>

I spent about an hour behind the router tonight snorting sawdust and reminiscing my college wood working class…
I made a good start on the inletting. Nice snug fit too.

This is what happens when a M-Rod (rifle) and a P-Rod (pistol) get together….
The B-Rod (bullpup)… yup it officially has a new name…
The scary inletting job is all done, it was a little more complex than I first thought so I’m very relieved to have that stage finished..
Another couple of hours behind the router and its starting to take shape..
I had to shift the lateral router line down about 10mm due to the thin wood in the scope mount recesses. I wanted to be well clear so not to
weaken it or in case they “broke through”. It hasn’t really changed the look to the original design though, they still look good.
I really wanted to stray away from the “chunky” looking bullpup designs commonly seen and endeavour to produce a thinner, slick, light weight
and more ergonomic stock.
I finished cutting the basic outline now and have started roughing out the contours.. Its starting to show some really nice grain now..
I’ve got it all shaped out now which only leaves the sanding part… lots and lots of sanding!
I’m really loving the lines of this stock, the grain follows the curves just as I hoped it would when
I laid out the template. I have tried to keep the Eastern European stile lines which I’m quite fond of.
Probably not every bodies cup of tea.. but then again nor are bullpups

I found another air leak due to a damaged barrel seal. The bolt has taken chunks out of it causing fluctuating velocities
over the chrono. I replaced it with a Poly 90 “O” ring which are much harder wearing and less likely to catch on the bolt and cut. Problem gone š

Couldn’t help myself… I put it all together for a sneak preview of how it feels and handles…
Here’s a pic of the B-Rod next to the Steyr for a size comparison, the barrel lengths are the same on both rifles.
Total length of only 700mm, thats 45mm shorter than the std Edgun Matador.

I simply had to test the B-Rod before finishing the stock and sending all the alloy bits to the electro platers to be anodized matt black.. It shoulders really nicely and the weight and balance is nothing short of superb! Very very easy to hold steady while standing, its going to be a great hunting rifle for sure. The trigger is very crisp and as I had hoped feels like any other regular adjustable trigger

The B-Rod got some attention this arvo.. sat out in the sun with my wife with a glass of wine and got to work sanding the stock.(Now that’s “Quality time”) One of my pet ducks showed up to provide the entertainment.. very hard case bird with a real personality! Follows me every where.. even hunting.. have to keep the house doors shut or he comes in to hang out..

I have sanded all the orbital marks and router burns out and worked the stock down to P120 grit. The worst part is done.. just got to work my way down through the grades now. I have wet it down with a wet rag and will let it dry over night, this raises all the loose wood fibres so they can be sanded off leaving a very good finish. I will do this with every step in paper grade until the final polish with steel wool and application of multiple coats of oil.

The pics below taken with a macro lens is the result of wetting the stock and drying it over night..
All the tiny broken fibres lift there heads for me to sand off making for a baby bum smooth finish and adds to the depth you can achieve when oiling timber.
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I had a lazy night tonight and took a rest from sanding.. Instead I drilled the stock mounting hole and made the trigger guard.
With the original stock mounting bolt now being to far towards the rear, the need arises for a new stock fixing screw. I have incorporated the new mounting screw with trigger guard bolt. I manufactured the alloy trigger guard in a soft smooth curve to follow the lines of the stock and it will be anodized black with the rest of the new alloy parts… please ignore the big ass s/s cap screw I have holding it on for the pic, It will be replaced with a nice flush mount black countersunk 1/4” cap screw on final assembly. The bolt threads into the base of the front scope mounting block (strategically designed directly over the trigger guard mounting bolt) and locks the action/cylinder securely into the timber. The trigger guard was recessed flush with the stock very carefully by hand with a chisel and hammer, in hindsight I should have done this near the beginning with the router while I still had square edges to work with. No probs though, disaster averted and no nervous slips with the chisel this time round š Hopefully tomorrow brings the enthusiasm to finish sanding the stock start the oiling process.
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well I did it.. got that job out of the way.. first sealing coat of oil on.
I’ll take some better pics tomorrow in the sunlight.. stunning piece of timber!

A total of 20 hours work and the stock is finished just applied the final coat of oil.



I have just fitted the recoil pad and weighed the stock, 980 grams.. the original Crossman stock weighs 1.450 kg so another weight savings on that one.


After another pre-fit in the stock I have decided to mod the trigger slightly,
I previously had just drilled small pilot holes for testing and was testing it with some small pivot bolts which worked satisfactory but I have decided to use an 1/8″ roll pin as the main trigger pivot point so I’ve added a larger 1/4″ diameter rolling brass bush through the top of the trigger so the alloy trigger isn’t wearing on the split in the roll pin. With a little graphite grease on final assembly is will be far smoother with a zero play super slick “roller bearing” feel to it. The trigger slot needed widening slightly to accommodate the new brass bush and now has a couple of machined shim washers to take up the end play. I have also decided to mount the trigger block on the front scope mount block only, trimming off a bit more fat and loosing a few more grams. It was a bit over engineered to start with I think.

Originally I was going to remove the pressure gauge and just blank it off but decided to leave it and make it visible through a porthole in the underside of the stock. Glad I did, Its made quite a cool feature out of it I think.
On the home straight now.. just got to drill and tap a few holes for the trigger to mount and polish all the machine marks out of the alloy bits to be sent away to the electro platers this week.

I finished all the final fitting of the trigger group today, I had to go with some very small 3mm cap screws to go through
the front mount to straddle the stock mounting bolt, tapping the tiny 3mm threads while the tap twisted and creaked through
the very hard aircraft alloy, It was a little nerve racking but no issues, I didn’t break it and it all fits up nice and square.

Another pre-fit with the new stock countersunk cap screw and its all sitting perfectly in place. I made a small guide for the rear of the trigger conrod that mounts to the old stock mounting hole with a shorter cap screw to keep every thing running square and smooth. Also turned a 4mm stainless steel pin for the trigger to conrod pivot, also fine tuned the rod length and trigger position. The trigger is silky smooth now and you couldn’t tell the difference between pulling the front trigger to pulling the std rear trigger.


Here’s all the new parts polished and ready to anodize. They collectively weigh 440 grams
Which means I have added 12 new components and the new stock to this rifle and it is still lighter
than the std factory rifle, pretty happy with that š

With all the bits polished and off to the electro platers tomorrow, I turned my attention to the accuracy issues this rifle was suffering when I bought it.
The groups were very inconsistent with 3 different brands of pellets I tried.
Not surprisingly the wolverine 25.0gr pellets made for BSA by JSB were the best shooters but at 50m I had plenty of flyers.
When I removed the barrel to start this project I pushed a few pellets through it with my cleaning rod, all seems very good, a firm consistent bite the entire length until the last tighter 2 inches where the barrel is choked. I would go as far as saying this barrel is tight, not uncommon for BSA barrels Ive heard.
Something didn’t sit right with me in regards to the bolt operation though..
It seemed to need far too much pressure to seat the pellet and lock the bolt closed. Tonight I looked at this a little closer by loading a pellet in then removing the barrel and pushing it back out the way it went it with my cleaning rod. The skirt was very damaged from the bolt and it had very uneven rifling grooves it the head, one side of the pellet they were cut deep and the other they were only very slight.
I repeated the same task with the original Green mountain barrel fitted and there was no issue at all and very minimal pressure required to close the bolt. This tells me BSA have a much tighter chambering/leads. I’m unfamiliar with the BSA Lonestars bolt probe system but its apparent that the Marauder is very different. Under closer inspection I found a very ugly chamber, the lead into the rifling lands was very lop sided, I would say as far as 2mm on one side, this explains the marks in the pellets heads and no doubt the cause of the poor accuracy. The uneven lands were acting like a feed ramp cocking the pellet to one side on entrance.
I had a measure up of the Green mountains chamber and compared it to the BSA barrel. The G/M measures 0.2600″ (6.60mm) and the BSA was only .2534″ (just over a 1/4 inch. Into the lathe it went.. I first opened it up with a 6.5mm (0.2560″) drill and tried a pellet.. It was even worse! so tight it buggered the skirt..
now how could taking material out of the chamber make it worse I thought? what it had done in turn was create a steeper angle on the rifling lands making a sharper step for the pellet to enter.. sooo… next I used a letter F drill, (.2570″ or 6.53mm) still not quite the 0.2600″ (6.60mm) of the G/M but thats the closest I had, and the next size up is a letter G drill, just a tad too big at 0.2610″ (6.62mm)
After running the F drill through it and pushing the lands back to the same depth as the G/M barrel I trailed another pellet. This time It was much better but it still had the sharp step onto the lands the require a little more bolt pressure than I would like so I ground a very large taper into the end of my F drill and gave the lands a light touch with that and gave the chamber a quick polish.. The end result is very pleasing, the pellet slips in with nice even bolt pressure all the way to lock up and now has nice neat even rifling cuts around the head.
Heres a photo of before and after.. Probably should have cleaned out the dust for the pic..

I also noticed the crown was very ugly and it had an off centre countersunk section, Hard to see in the photo but that certainly wasn’t aiding accuracy..
Back on the lathe I cut one of my favourite target crowns.. a square cut crown with an 11 degree target recess, mainly because its pretty but also very functional.
Hopefully once its all back in one piece with this barrel work done along with all the replaced seals this little pup will stack pellets with the best of them.. time will tell…:rolleyes:
[COLOR=”DeepSkyBlue”]Before:

After:

With the B-Rod build nearing completion only awaiting the return of the newly anodized parts and final assembly, the hunt for the perfect .25 cal pellet to short BSA barrel marriage startsā¦.
Instead of using 10 different pellets and the related ideal velocities for each, the Marauder is to be set at the best velocity for the best pellet. In the coming weeks, I’ll share my experience in determining that pellet and touch on rifle tuning techniques.
Unlike my Steyr the Maurauder is unregulated meaning the velocity and cylinder pressure work independently
If either one of these variables changes, the other will also change. When setting a non regulated rifle up, you have to find a balance between both variables. It isn’t difficult to do, but it’s a complex relationship. A chronograph is defiantly needed to adjust the velocity.
There are 3 main adjustable components on the B-Rods air system:
The velocity metering screw (opens and closes the port into the barrel), the striker pin valve travel and the hammer spring preload.
Ideally I want a heavy pellet 28-40gr with a minimum of 50ftlbs,
I intend to use the B-Rod for short range goats and long range rabbits just how ālong rangeā will be determined by the capabilities of the selected pellet not the rifle I’m guessing..
The pellet needs carry plenty of energy short range and be very accurate at the longer ranges so I’m focussing my testing on proven dome head pellets with the higher B C.
Like my Steyr I’m going to run the bullpup very hot to get the most energy the .25cal has to offer, an accurate flatter flying pellet is more important than shot count for me.
I have no interest in the gimmicky hollow points or pointed pellets as the expansion on the hollows is very marginal and the B Cās suffer on both types limiting their long range capabilities. I do however have a part tin of H&N hollow point that came with the rifle when I bought it so I will add them to the list for interestās sakes only.
I have spent quite a few hours searching for info on the net for related .25cal ballistics and found very little facts on many of the pellets that I will test.
There was some good info on heavy cast bullets also Eunjin 35.8 and 43.0gr pellets seemed to be favoured in the states but neither of these seem to be overly accurate or available here in NZ. I found some on eBay for 8 pounds and 2 pounds postage but I just canāt seem to bring myself to import and shoot Korean made pellets no matter how cheap they are.
One pellet that did get my attention was the Daystate Rangemaster 36.0gr pellet, this one ticks all the boxes for me but they were unavailable unless I imported them myselfā¦

So I Did⦠š :rolleyes: I cant wait for that package to turn up..
The first thing I noticed while purchasing test pellets was the very limited range and stock of .25 pellets available in comparison with .177 and .22 pellets. Some gun stores didn’t even know what they were and tried selling me .22 pellets as they were sure I had made a mistake.. They also seemed to lean towards the light weight side of pellets. However⦠I have imported some myself and I rang retailers from one end of the country to the other and scratched together 11 different hopefuls:
JSB Exact Kings 25.4gr
H&N Field Target Trophy 20.06gr
BSA Wolverines 25.0gr
H&N Barracuda āAKAā Beeman Kodiaks 31.02gr
Benjamin Premiers 27.8gr
RWS Super Domes 31.0gr
H&N Hollow Points 26.23gr
Predator Bigboy Senior 31.0gr
Gamo Hunter 21.6gr
Daystate Rangemaster 36.0gr
I will start by shooting all of these pellets with the gun set on the āMaxed the fk outā settings, pushing them as hot as they will go to see if any of them will group..
From there I will fine tune the hopefuls.
I will review each pellet in depth as I go and with any luck get this M-pup shooting inside the inch at 100 meters
All the parts are back and black, I will assemble and post picks tomorrow..
For now here’s some info on the current state of tune..
The valve
The most commonly used air valve in airguns today is the knock-open valve. To operate it, a valve stem is struck by the hammer that is driven by a spring. The inertia of the impact moves the valve stem in the same direction. There is a synthetic valve face on the valve stem that holds the high-pressure air (or CO2) inside a reservoir until the valve is open. A valve-return spring starts the valve moving back toward the closed position after the valve has opened as far as it can. Once the valve stem starts to return to the closed position, the pressure of the air or gas in the reservoir also pushes on it. That’s because the pressure inside the channel of the valve body is always a little lower than the pressure in the reservoir. If it weren’t, the valve would remain open longer, dumping a lot of the air in one shot. The little valve-return spring exerts a huge controlling force over the valve because of this pressure differential.
How the valve is designed and set up determines performance
You can affect how long a valve remains open by adjusting the strength of the return spring, the pressure inside the reservoir, the length of travel of the valve stem and the inertia of the hammer. The more dwell time the longer the valve remains open, the more air will flow through it. There are limits, however, which is why you don’t get an increase in power beyond a certain point when you install a heavier hammer or a stronger hammer spring.
I have replaced the factory 8 lb hammer spring with a 10.5 lb spring and set the dwell time on the valve striker pin to it shortest setting meaning its open for a shorter time. Fitting a spring to this rifle over 10.5lbs wont change anything it will only make it harder to cock. Some rifles will need a mod to the transfer port after a mod like this to cope with the shorter burst but greater volume of air but not the Marauder, it has an adjustable metering screw which opens and closes the transfer port. I have set this screw to the maximum open position.
The heavier spring also eliminates hammer bounce which waists a good 20% of the air in the factory settings with a weaker spring fitted.
With the mods I’ve done to this rifle I substituted dwell time for the extra pressure, to extract all the performance the air has to offer in a shorter time.
The secret with this guns inherent accuracy is a well-balanced valve that doesn’t require raw pressure in order to function.
The Marauder is set to operate on a 3000 psi fill from the factory. I say that because the Marauder will function with any fill pressure from 2,000 to 3,000 psi ā itās adjustable by the owner. But the .25 screams to be set up for the full 3,000 psi. Thatās because this big .25 is a real thumper that uses a lot of air for each shot.
However, while I was testing the gun on factory settings, I fired strings starting at a 3000 psi fill. In other words, I topped up the gun between each string. In doing so, I noted that the initial shots were lower in velocity, meaning that this particular rifle needs a fill of somewhat less than 3000 for optimal performance. It got 16 (two mags) very consistent shots between 2800psi and 1900psi before the pressure drops away rapidly. The factory set it up this way to get the highest shot count and smoothest shot curve possible.
The mods I have made to the B-rod to make the most of its 3000psi fill but subsequently have sacrificed shot count for velocity and consistency, in my application this is what I wanted.
Check out the curves below to see the results of the tune up.
The red line is factory setting and the green is after the tune.

Red * I shot strings from a 3000psi fill in the factory settings first, the 16 usable shots between 2850psi and 1900psi over the chrony were as follows:
The pellets were Benjamin 27.8gr Premiers
773fps 796fps 802fps 798fps 784fps 793fps 811fps 801fps 821fps 826fps 829fps 805fps 821fps 817fps 802fps 789 fps
Average speed: 804fps
Average energy: 39.9 ftlbs
Extreme spread: 16fps
Standard deviation: 15.7fps
Not bad for an entry level hunting rifle!
Green * The second shot string with the Benjamin 27.8gr Premiers
from a 3000psi fill to 2000psi is the 10 usable shots after the tune.
The chrony results were as follows:
958fps 957fps 955fps 956fps 957fps 952fps 948fps 946fps 945fps 942fps
Average speed: 951fps
Average energy: 55.9 ftlbs
Extreme spread: 16fps
Standard deviation: 5.8fps
Now those are the figures I was hoping for⦠on to the pellet tests and groupings…
The parts are back and the anodizers did a perfect job for me as usual…

I fitted it all up with nice new cap screws, graphite grease on all the trigger mechanism and silicon grease on all the new seals. Its all looking and operating so sweet..

Here’s what you have all been waiting for…The Marauder bullpup “glamour shots”
It is one fantastic rifle for such a minimal cash outlay.


Here it is with some larger Falcon optics fitted, I decided it was just too much scope for the B-Rod, it added too much weight..
After a couple of sighters I gassed it up and emptied 2 mags of BSA wolverine pellets onto paper.. I must say I’m still grinning!
One ragged hole at 30m! Cant wait to trail the other pellets and stretch its legs a bit further!

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http://www.ginb.it/prodotti_list.php?lang=en
Scroll farthest down on the page for BP stocks for various inlets…
yea I looked at one of those GinB bullpup stocks years ago for my Steyr but decided against it. they are all very chunky and would need some fat trimming off to look right IMO. I would love to take a look at the trigger kit they supply if any one out there has one of these bullpups?
I didnĀ“t like the look of those either but they do make your stock as you want it provide you use their standard BP stock as a starting point. I quess the price would be high nevertheless and since youĀ“re able to make your own stock exactly as you like the only interresting part for you would be the trigger assy – or maybe itĀ“s just a worthless afterthought. IĀ“d like to see the trigger kit as well.