.308 blowup
Fired my .308 again last night, same result 🙁
For those of you who don’t know, I’ve been trying to blow myself up for awhile now with moderate success.
The breech doesn’t want to seal up against the valve tophat. A single O ring, 5/8″ OD and 1/2″ ID, 1/16″ cross section, kept blowing out; the ring actually extrudes into the space between the breech and tophat, which on my breech is about 0.0015″
I decided to cut a secondary O ring groove and place another O ring in front of the existing O ring since there was just enough space to do so… this was last night’s test, to see if the backup O ring would hold the pressure. No dice. Both O rings blew out. And yes, my ears are still ringing. I have 10 fingers left.
Did a bit of research on the net and found that the Buna-N (standard Nitrile O rings we all use) are normally 70 durometer and rated for 1500psi… all the charts I’m following with the groove dimensions were triple checked and my grooves are correctly dimensioned… but it seems the o rings themselves weren’t meant for this kind of pressure. Found one interesting graph which showed ring hardess, or durometer rating, vs pressure and gap size… if you go down to near .0005″ on the gap (breech to tophat radial clearance), the working pressure can go much higher, as high as 10,000 psi. Also, going to 90 durometer also ups the allowable working pressure. But, by far, the most interesting thing I found was that the standard nitirile O rings are very susceptible to extrusion; they like squeezing through small spaces under pressure. But there are many other o ring compositions, such as Viton and especially Urethane, which have several times the extrusion resistance of nitrile compounds. I just ordered some Urethane orings and will have to await these before attempting firing again. For anyone else interested in Urethane o rings, try http://www.marcorubber.com. I also found them at Grainger.com, but they only had urethane in 70 durometer; marco had 90 durometer urethane. In my size, they were .83 cents each and there is a 30 piece minimum order.
While awaiting the o rings, I’ve decided I should take my barrel out and taper the leade inside the chamber. I think the abrupt leade is causing too high a pressure inside the breeach area before the rifling becomes inscribed. I used a standard .308 match reamer to cut my leade… but after inspection, a powderburner leade is very abrupt compared to an airgun leade. Anyone out there already made a .308 cutter that I can borrow??? If not, gonna have to make a long tapered cutter for this job… I’m thinking at least 1/2″ or so with a very, VERY gentle taper in for the rifling to slowly take hold of the bullet ogive.
aloha,
walt
All Replies
my post was to mention that using a powderburner barrel can be done, and with just 10% sacrifice to power, and that but polishing a powderburner barrel you can make a .308 barrel say a .309 or .310 wich reduces the rifling grooves, as well as giving projectiles that in .308 often are .310 less resistance and therefor more power
my .308 project topped out at 249 fpe with a 119 grain projectile of purelead
the powderbunerbarrel was from midway and 65 bucks for a blank, i could consistently hit a 12″ disc at 200 yards with it…..but i do reommend that you make the barrel front heavy, the AF guns are very butt heavy, and when upping the power to 100fpe you get quite a bit of muzzle rise….so adding a brake or weight out there in front really helps
this leads me to another VERY important powerfactor….lead….softlead gives the best results imho, both in terms of power, but also in killing ability, the hardcast stuff sucks alot of power to push through the barrel, and at the speeds airguns are going, they dont deform unless you hit a a hard surface like metal or rock….bones are no problem, they push right through
i have seen as much as 30% power difference in just choosing the right projectile
secondly because of the power…i suggest a 1 piece bushing…i tried this as i was having a real hard time with a barrel i had machined down too far, i think it was able to flex in between the 2 bushings i was using, even adding a 3rd bushing in the front of the frame didnt help…so swithing to a 1 piece eliminated that flex, and i didnt have to have the 3rd bushing
found an old .308 string
Grain ➡ FPS ➡ FPE
119 ➡ 872.49 ➡ 201.2 308 for sale, powerwheel at 6, 230 bar
119 ➡ 864.7 ➡ 197.6
119 ➡ 846.44 ➡ 189.4
119 ➡ 842.2 ➡ 187.5
119 ➡ 820.06 ➡ 177.7
119 ➡ 807.69 ➡ 172.4
119 ➡ 797.53 ➡ 168.1
119 ➡ 781.06 ➡ 161.2
119 ➡ 774.37 ➡ 158.5
119 ➡ 750.88 ➡ 149.0
45 ➡ 1156.37 ➡ 133.6 refill to 230
76 ➡ 969.48 ➡ 158.7
83 ➡ 944.52 ➡ 164.5
Cygnus, I believe it. The DAQ barrels are much, much smoother and finished better. The Mauser barrel I have, in comparison, is very rough; so the extra smoothness of the bore plus the fact that the rifling is shallower for less blow by of propellant gasses mean more velocity for sure. BUT, the mauser barrel was free… I bought a bunch of old 1895 Mausers and only needed the actions for other projects, so took off the barrels and had them lying around. I’m not too concerned with final power output, the amount of noise it makes is a larger concern to me. But good going on your 220fpe!
aloha,
walt
i used a .308 powder burner rifle barrel and got about 200 fpe out of it….then switched to a DAQ .308 barrel and got 220 fpe out of it, same ammo, same valve, same fill pressure, same breech
so 10% more power from a real airgun barrel thats hard to come by, vs firearms barrel thats in stock alot of places for a fraction of the price.
if you really conserned about the rifling, one could allways pull say 500 patches with a polish on it through the barrel to remove .001″
thick orings vs thin ones….the ones in the breech from me is 90dm, but i have used 70 as well, and sometimes they would blow out, a little depending on caliper….cause in .308 project i had i used 70dm for the most until i bought a 90dm assortment from mcmasters
Picudo, you have inconel??? Man, I’ve been trying to get my hands on some of that stuff for awhile, my usual suppliers of 301 and 6061 can’t seem to get any. I have a hunch even if they could, it’d be far too expensive to really play with. You’re a lucky man!
Brad, the 10/22s suffer from a very vague barrel attachment system that allows a bit of creep under differing temp changes or with shock if you bang it around. If you really gorilla the locking lugs you can get it reasonably secure, but some people are resorting to epoxying the barrel into the insertion point in the receiver and between the lug and receiver to make sure it doesn’t shift. To get around this entirely, it’s easier just to mount the barrel the same way you do any gun barrel; by threading barrel and receiver and locking them together in a class III fit. No more wandering point of aim, guaranteed. Smiths who specialize in 10/22’s have been doing this for years. But, it’s not for the casual 10/22 owners; you’re talking custom everything, including aftermarket receivers in steel, not the stock aluminum; and whole aftermarket trigger groups with really tight dimensions. Most folks would shake their heads and walk away when they see the prices attached to this stuff, you could get a Stolley action and build yourself a really nice one hole custom .308 for the same price.
Is there a reason you want to thread the receiver I have been a gunsmith for a long time and built a lot of custom guns I never heard of anyone doing that to a 10-22
Walt
What we do sometimes is “guide” the thread (that is what we call it here, don’t know whether there is a real name for this) and then use the tap to complete it. We take very small cuts with a boring bar, just enough so that the tap will run true and centered. We have done this on Titanium, Inconel and other almost impossible to thread materials. Don’t know whether the lug will still be on the way of the tap or not, but it is an idea you may consider.
Good luck 😛
Hey Brad. Busy here, sorry. the barrel is just what I had lying around, it was from an arsenal reworked ’95 Mauser in .308 that I took apart. The barrel seems in good shape and the rifling is 4 groove and seems to be around 11:1 twist, which works fine. I put one of these barrels (recontoured, of course) onto a DAQ pistol and it works wonderfully. One hole groups all day long. The rifling is cut a tad deeper on the Mauser than Dennis cuts his; but only marginally, by eye it looks to be .002 deeper or thereabouts. For sure it isn’t as smooth or finished as well on the inside grooves; but then again, this old steel is HARD, not like Dennis’ stuff. Ought to last far longer than me, for sure, someday someone will take this thing apart and wonder who the hell put it on there. I acquired a big box full of old, old tools from a junkyard which had accumulated 70 years worth of cars and old junk and sorting through all this stuff, I sat there for hours and hours just shuffling stuff around and looking at the wear on some of these old tools and wondering about who had these in his hand and what he was working on so long ago; pre WWI, some of it. I could see initials and names carved here and there; some tools I didn’t even recognize, had to guess at their purposes. Neat stuff. I keep wondering what my son will think someday when they sort through all MY junk. We all have them; little bits and pieces of stuff from years gone past. Little one-off tools made just for a specific job; to adjust just that one valve you couldn’t reach in that old car you no longer have; or to take apart that gearset with the odd pin hole placement; all these special things you had to make for that one job, and when you go, who will remember where they go, what they do, their purpose for existance gone? Sad, that big steel box was so full of memories like that except I had no way to access any of them. Ghosts hovering over a bunch of well used, carved up hand tools.
lol, he dont like you!!
sounds like a sweet project cant wait to see resaults
Walt are you going to respond to my question???
Picudo, yes, that was me on Rimfire Central 🙂 No leads yet on a high quality .22LR barrel (used/cheap), I’m looking for a heavy stainless barrel or two. Easy to thread the barrels, not so easy to thread the 10/22 receivers due to the big lug that sticks out from the front face of the receiver that you normally bolt the barrel lug into… when threading the receiver, I’ve thought better to just cut this lug off, otherwise I have to make a long tool to stretch past this lug to cut the threads; but unfortunately, the lug underside also has the threaded hole that you attach the stock with, so if I cut this off, I’d have to weld it back or otherwise figure out how to reattach it at exactly the same spot. Ugh.
I’ve thought to just use a tap instead of single point cutting the threads on the lathe; but instead will probably just make a HSS bit to put into a boring bar with a broach cut holder and use that. Easiest way out, I think. Except my receiver is stainless, which work hardens quickly… so cutting internal threads with a single point tool is going to be a real pain, even more so with the tool hanging waaaaaay out there at the end of a long boring bar. I’m not looking forward to it; may just make a jig to locate the big lug correctly and cut the damn thing off and then weld it back on.
If you have any high quality stainless heavy barrels in .22, let me know, I’m interested. I also need a .22LR chamber reamer; Bentz or near that class.
aloha,
walt
Walt
Saw your post in Rimfire Central, if that is realy you, how’s that 10-22 comming? have you found a way to thread the receiver yet?
cheers!!
Cygnus, you ought to know…. I’m using your valve, after all 🙂 I thought of the O ring on the barrel side of the breech as well; but so far it hasn’t given any trouble. I chose to cut the same OD O ring on that end, but chose the next thicker size; it is like twice the thickness and very stiff due to its thickness, even in 70 durometer. It makes it harder to open the breech, it gets really stiff and hard to move; but worth it in security. Since the polyurethane O rings are so expensive and hard to find, I will leave that one as it is for now and see if I have trouble blowing that one out after I fix the valve end.
I usually am very careful about new gun builds; I use the string and sandbags too, especially after installing a new barrel on a 308 or 30-06. But I figured this 308 airgun wouldn’t have nearly the same energy, so I haven’t done too much precautions other than wrapping it in towels before firing just in case something lets go. Don’t want high velocity aluminum pieces flying…
i have used 90 durometer with success, 70 dm will jam, say on the barrel side of the breech its likely to squeese out and bind the breech a bit
also its very important to keep the breech from rebounding, when the valves of high power shut close they do so with speed and force, enough to launch the breech right off the tophat, then the air can blow the oring out of its groove with ease, and it might get jammed out of groove and the tophat
but yes, a blown oring close to the breech will make a loud noise, i usually testfired with the rifle in a wise, and a piece of string to begin with….and then would move on to the toilet paper test…put it over the breech if it blows off u got a problem…if it just flutters your prolly just fine…test fire alot before moving the rifle up to your head…note down how far you can shoot it before it dumps etc……
Hey Walt.
Ive been using urethane 90+duro rings in all my guns with great success. Usually can find whatever you need at McMaster pretty cheap.
They are stiff though, not easy to get back in a groove sometimes. 8)
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Good luck with your project Walt!
Mike