Vickers not running well in 7.62x54r?

[glennp]

Hello!
I'm new to the Vickers MK1 and I have a new to me transferable that I've had the pleasure of taking out twice so far. The conversion consists of a reamed out .303 barrel, a modified extractor, and modified feed block. The modifications were done by known people and appear to be of good quality. The rims clear the feed block and extractor smoothly. I actually have two of each modified for the barrel, feed block, and extractor. The first outing, I took 500 rounds of .303 and 500 rounds of 7.62x54r on PKM belts. The gun ran the .303 nearly flawlessly. Then I changed the barrel, feed block, and the lock for the x54r parts. I left the booster cup off. It didn't run well, it would run 3-15 rounds and then have a fourth position stoppage. The ammunition I'm running is Wolf "military classic". I figured it's a lack of oomf, so I tried lightening the fusee spring. I got it down to about 5-6lbs before I gave up on that approach, knowing that's quite light. The cyclic rate was higher than .303 so it certainly had the "poop" to run, but would still give me a fourth position stoppage frequently. That is, no round in the chamber, but a round would be in the extractor ready to go most times(one charge to get it going). Then I switched out one by one a different barrel, then a different lock, then tried a different feed block. Zero effect!

When I took the gun apart at home, I noticed that my new packing was pretty tight and goopy (graphite yarn). I figured I had it figured out, so I cleaned it up and put it away for the next outing. Fast forward to my next outing, I limped through 750 rounds and getting the same malfunctions. The gun would stop running, but 9/10 times one pull of the crank and a yank on the belt is all it needed to spit out the next dozen or so rounds. Quite perplexing. I even took slow-motion videos of the feed-side of the gun to see if I can see any difference with when it quits, but I cannot. It appears to be fully recoiling and yanking the belt in.

I did notice on the second outing I am getting a couple case-head seperations which are of course a different ball of wax. I of course forgot the shims at home, but I do understand I need to shim under the lock stub to push the lock forward more in relationship to the barrel. However, I don't think that would be causing the more common woe, which is frequent 4th position stoppages.

What does the brain-trust think?

[Countryboy77]

Was somone tailing the belt? All the really old guns are finicky and take a bet to get running.Headspace change between swaps maybe.Not a lot of time on a vickers,but played with my friends.