308 conversion question….
I’m starting the machining on my 308 barrel; where the barrel slims down to slide into the breech, how slim do you guys make yours? I’ve only owned my condor a few days so don’t have much experience with them, although the design seems simple and straightforward.
The stock .22 bbl was around .309″ OD at the breech, so the walls were about .043″ thick at the breech. I was thinking of just about doubling this to use a wall thickness of around .080″ (yielding a breech of .468″ for a 308 conversion)… I’m aware that making it too big will waste air as this is the propellant chamber, so volume should be kept as small as possible to keep the gun efficient. Am I in the ballpark with a .468″ breech? I haven’t done the O ring calculations yet to see if I have some O rings in this size to go into a new breech that will have to be fabricated… I figured somewhere between a wall of .080″ and .065″ on the slim side; somewhere in between should be a suitable O ring size.
aloha,
walt
All Replies
Viewing 11 replies - 1 through 11 (of 11 total)
Why not just use a powder burner?
Thanks YN. Actually, I bought CygnusX’s stuff and it just arrived in today’s mail so I”ve been looking it over. I’ll be using his valves in my conversion, but I think my needs aren’t that critical since I only need enough power to put down wild hogs cleanly; let’s say around 150 fpe or 200 fpe is really all I need, not the gonzo 600fpe he was pushing with the same equipment. My DAQ puts out only a little over 100 fpe and the pigs drop immediately when headshot, so 150 to 200fpe would make no operational difference, although I might have to get 2 guns and leave one as a 45 and one as a 308 since I don’t want all this stuff to go to waste.
aloha,
walt
I used .223 FMJ’s in a .308 Plastic Sabot for testing… 68gr and I managed to get 104 FPE.
Thats without any valve modification, just changing of springs and tophat height. I have to say my gun didnt get many shots and I think after 104 fpe it went down to the 80’s.
I also had some .308 rounds from hunter supply that are sold for the DAQ .308’s. But those never went above 90fpe.
I ended up trading the barrel and going with .25… 20 shots all above 80 fpe and accurate as heck with no need for a custom breech. I HIGHLY recommend just going .25 unless your going to do valve work to increase power – Cygnusx was able to get well over 200 fpe from his .308 iirc.
Thanks, Marc. I’m using the Parker O Ring manual, available as a PDF download… http://phonon.phys.unm.edu/tiki/tiki-download_file.php?fileId=67
Table 4-6 seems to be the most informative. Use the picture key a couple pages before it to identify the dimensions. However, it gives dimensions in SAE, not metric; sorry, us US blokes make life hard.
I’ve used it in prior projects and it seems to be pretty much on spec. However, I’ve always made male gland applications before; this will be my first attempt at a female gland (internal O ring).
I haven’t decided yet, still crunching numbers. Female glands are hard… you have to be right on the money. With malee glands you can just stretch the O ring a bit if you are between sizes. I think female glands can be stretched 4% upward at a max to get between sizes.
walt
PS: I have been using my DAQ 308 pistol with 121g ammo and with its 12″ bbl have been getting around 625fps.
What bullets do you guys shoot in .308??
And what kinda velocity do you get?
YN’s way is the way to go. For instance If you have some o-rings 10*2mm
Machine the barrel 9.95 the hole in the breech 10.05. Then cut your o-ring grooves to 13.95 to 13.9mm. That way your o-ring will slide freely and close up the gap when pressure is applied.
Hope this makes sence.
Regards,
Marc
Well, since you have to build a custom breech, I would machine the breech first, install o-rings in it, then see what they measure installed. You then have your diameter to machine your barrel to.
I never done it myself, but it sounds logical.
Thanks guys; just what I needed. So Marc, your 11mm mics out near .433″… close to what I came up with. And YellowNinja’s by eyeball looks like a 1/2″ OD, and maybe 1/2″ ID on the O rings? So close to .100 wall thickness. Great stuff, thanks. I’ll go break out the O ring case now and see what I have lying around….
aloha,
walt
I made mine…. this big –
I dont beleive I ever measured it…. I just went to the hardware store and got O-rings bigger than .308 and based it on that… Would suck to machine it and then find out there isnt an O-ring in that size easily available.
I used 11mm on my 308 conversion. But I think you migt get away with 10mm.
Regards,
Marc
Viewing 11 replies - 1 through 11 (of 11 total)
- You must be logged in to reply to this topic.
Several reasons. I have lots of powder burners but it’s not a challenge to kill a pig with 3000 ft-lbs of powder burner. The airgun is a much more elegant tool. Also, some of this shooting is done in people’s back yards to eradicate pests and some associations have rules about dischargning powder burners. And even if they didn’t, it’s just too loud to use when the neighbor’s house is only 20 ft away.