.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