Opening up a chamber?

[stencollector]

I am having a bit of a problem with my new conversion on a TNW M1919A4 semi auto belt fed gun. I got in a 8mm barrel, and had the chamber reamed out for the 8X63 swedish ammo thats available. Took the gun to the range on the weekend, and it worked better than it ever has with one exception: out of about 500 rounds I had 5 or 6 hard extractions. The empty case would stay in the chamber and the gun would try and feed another round into the spent primer.

No problem removing the live round, but getting the empty case out of the chamber was a bit more work. The second day I brought along a cleaning rod, which made it not too bad, but not the kind of drill I would want to be performing during a shooting match.

For the second day, I tried a smaller barrel bearing, hoping that the extra backpressure might help, but it did not. I had about 5 stuck cases out of almost 500 rounds fired.

I am thinking about opening up the chamber a bit. When I compare a live round in the chamber of this barrel against a live round in the chamber of either a 308 or 30-06, it does seem to have a lot less play to it. Remember that with the GPMG we are talking quantity vs quality. I was going to simply run some valve lapping compound into the chamber, and work it with an empty case attached to a drill. So what do the pros think? Will this help, or am I wasting my time? What about merely increasing the headspace on the gun? I went one or two extra clicks on the barrel adjustment, but the results were the same.

Any advise is greatly appreciated.

 

[gunpro]

is there any differences in the brass (deformity, scatch etc) of the rounds that got stuck versus the ones that worked fine? I am thinking if it was a chamber issue you would be having way more stoppages and jams than roughly 1 in every 100 rounds or so.

Hate to ask but how clean is the chamber? is there a build up that is catching your brass.

My gut is leaning more to inconsistent ammo but i have not seen the gun nor fired one of these before. Maybe someone else with more milsurp experience on this model can share some insight.

IF you wanted to go with the chamber lapping, i would use some 600grit lapping compound and do it by hand, skip the drill. any off-centre and you could potentially ruin your chamber. Then clean the snot out of it or it will keep working.

 

[stencollector]

There is some inconsistency with the brass before firing. It is 1941 period ammo on cloth belts. It was also suggested that perhaps after 65 years on the cloth belts, the brass may have been dragging a bit of fabric in to the chamber with it. The variables on the brass are also what is making me lean towards opening the chamber....seems to me the military usually had slopier tolerances to allow for this kind of thing.

The cases which got stuck don't show any obvious problems other than the edges broken off the case from the bolt. I put the caliper to them, and compared them to brass that came out fine, but perhaps I will explore this avenue some more.

Since the cases are tapered, would adjusting the headspace a bit more give similar results (within reason) to opening up the chamber?

 

[Andy]

Have you compared the measurements of unfired brass to fired? If there is little difference you might have a minimum spec reamer and thus opening the chamber up a bit (or at least polishing it) might be the answer.

I have shot quite a bit of this ammo in a bolt gun, and find about 2% split necks due to brittle aged brass. Are the necks ok on the ones that hang up?

 

[tiriaq]

I wonder if the reamer that you used is identical with what the Swedes would have used in a barrel for one of these Brownings. No harm in polishing the chamber, without altering dimensions. Are the rounds clean coming out of the belts? No residue or hint of corrosion on them? Is there any possibility that the breech is opening when there is still residual pressure in the case?

 

[stencollector]

Andy: I also found a small percentage of split necks, but not on the ones that got stuck. Necks are OK on the ones that got caught up.
The guy who re-chambered it for me also reported similar hangups. He has the same brand of chambering tool that you have. He also reported finding cracked necks on some rounds that hadn't even been fired yet. Since we are not going to reload, it doesn't much matter, as long as we get reliable firing.

Tiraq: I thought it might have been insufficient gas the first day. The opening of the barrel bearing dictates the cyclic rate of fire. I was using a bearing that had been opened up to 3/4". An origional 30-06 is open to .716, and a .308 cal is bushed down to 5/8" . The second day I went out, I reworked a bearing to 11/16. Things weren't better and they weren't worse, so I will go back to the 3/4". I may even work it more open, so the gun doesn't take such a beating. But with the Manitoba climate, I suspect that in the winter, more gas may be needed so I don't want to push the envelope too far.

Rounds are clean overall...there may be some cloth particles going in with the belt, but I can't be sure. Next time I go out, I'll pay more attention and mark the belt at the points where the gun jams.

 

[buckbrush]

The Browning has adjustable headspace. Try adjusting it to more headspace.

 

[tiriaq]

If the case is staying in the chamber, and the bolt has enough energy to pull through the rim, I'm wondering if reducing the backthrust might not slow things down and give the case a little bit more time to release. Less energy, rather than more. The 8mm will have more muzzle gas than .30-06, so would it be worth trying a bushing even bigger than .750?

 

[stencollector]

I compared the barrel to my one 30-06 barrel, 3 CAL Cdn issue 7.62 barrels, and an Isreali 7.62 barrel. On all the other barrels, you can bottom a live round into the chamber, and then pull it out with no resistance. On the 8mm barrel, you have to push the round in for the last fraction of an inch, and then pry with a baby screwdriver to remove it. So I went with the tight chamber theory and lapped the chamber. I still have the slightest bit of resistance, but nothing like before. By testing with a live round, and turning it in the chamber, I get a slight polishing now on the front neck of the case, so I lapped that area a few more times, and think I'm in the ballpark now. I'm going to take it back to the range tommorow morning and run another belt through it. Lucky the ammos cheap.

I'll also try a bit larger of an opening with the muzzle bearing. .716 was the origional size for the 30-06, so I would have to imagine that at .750, I am in the ballpark now with regards to the size.

If I do get stuck cases again, I'll also try and back out the headspace a few more notches until I start to see bulged cases, and then run it back in a couple clicks.

Any suggestions as to how to polish the chamber? I am a shadetree mechanic, so any suggestions have to take that in to account. The lapping compound has left the chamber somewhat greyish.

 

[stencollector]

Well, today I ran some jewellers rouge onto a pointy little buffer attachment which goes onto the dremel. That helped put the shine into the chamber.
Now a cartridge will drop in to the chamber, and fall out when the barrel is inverted. The cases with the broken rims wil fit in with a bit of pressure, but still have to be worked out. They are not nearly as severe as previous. Of course, it is raining here in Shilo, so test firing will have to wait a day or two.

 

[stencollector]

Got it out to the range today, and it worked flawlessly. Only stopped twice, and both times it was because the belt was empty. I had it on the Jeep mount, so it works out nice in that the emptys all fall to the floor, and you get to sit down while you shoot. I also built a tray to hold the cardboard box of ammo coaxially with the gun: this worked very nicely as well. This thing is way too much fun to be legal.

Anyway, thanks for the tips and suggestions.

 

[Andy]

Great news. I'm trying to find an original 8x63 ammo can for you.

I think I'll buy another thousand rounds as well. Based on your burn rate they won't last long. A couple of years down the road we'll be remembering the good old days when Marstar was selling it.

 

[stencollector]

1500 rounds in a week....yeh, at this rate I'll be running out in about September and wishing I had bought more. I'm thinking of another 10,000.

By the way, while the gun ran flawlessly, the cracked necks certainly didn't get any better (not sure it got any worse either though). Looking at the emptys, my ratio of cracked necks is considerably higher than 2%....it's probably closer to the 20% rate. When I inspected one of the belts before hand, you could certainly see some cracks right at the end of the rim. I am not about to remove them from the belt to inspect any further though...they only have to shoot once to make me happy.

A source for the can would be great. Something about a cardboard box mounted next to the gun just doesn't look rugged enough. I may have to design a match here in Shilo that has a battle run, so I can shoot on the move.