Pretty sure this is because I'm using 147gr flat noses.Originally Posted by LittleLebowski
Pretty sure this is because I'm using 147gr flat noses.
#RESIST
FWIW my Z5-P or whatever it was (MP5K-PDW clone; threaded and 3 lug barrel) that I SBRed has not had a problem with 147 flatnose, ball, JHP, etc. I replaced the locking piece with an 80 degree one, replaced rollers to bring bolt gap back to tolerance, and no problems after that. The can / mount I'm using is an Omega 9k with the older/larger 3 lug mount (that's external to the can, not one of the new ones that mostly rides inside the can), and it's fun as hell.
The other thing you might want to do - if you had misfeeds of any kind, replace the extractor spring. They go out of wack REALLY easily on the 9mm guns as soon as there's a misfeed. If you want added insurance, replace it with a G3 rifle extractor spring; it's a lot stronger. It's probably overkill but it also won't get bent and become useless after only a couple of misfeeds. The extractor spring on 9mm MP5/etc is only as strong as it needs to be for a pistol cartridge (typical German, only as much as necessary), and replacing it with the rifle version of the spring can help.
For those going "What? Huh? Bolt Gap?" - order one of these - Feeler Gauge Set from Grainger. You can order the "official" (maybe) go/no-go feeler gauge set from HK Parts...which has two gauges, and costs $45. Or you can order this one, which does the same job except better, and it's cheaper. Hell, buy two for the same price as one of those. If you already have metric feeler gauge sets in .05 mm (or better) granularity, then rock on.
HKPro has a good write up and I think this is a copy - https://www.reddit.com/r/MP5/wiki/guide/bolt-gap/. You want to get your bolt gap as close to the maximum as you can - as the parts wear, bolt gap will close over time. It might never get into an out-of-specification distance but it will shrink, so starting from .45mm is a good thing, not a bad thing.
To measure, just unload the gun, lock the bolt to the rear, slap-chop your way to success, drop the hammer on the dry gun, invert the gun, and insert a feeler gauge between the bolt head and the bolt carrier itself. The .25mm leaf should fit in and bottom out just fine; if you're going for max gap, .50 should barely scrape in and anything thicker should be a hard "not fitting". If .50 or over drop in, you need to find a set of rollers (RobertRTG is a fantastic resource and pretty reasonably priced) that are smaller than your current roller markings. Personally I'd aim for .45mm as max, but .50 is within spec. It'll just feel a little more "robust" at .50.
I'm not sure what less than minimum gap feels like, but on a K3P/G3k clone-type toy, having a bolt gap of .55-.60 (slightly too big - .60 didn't fit but .55 did) makes it hard to unlock and rack the action, and man does it kick like a mule. After two mags it was clear it was NOT closing the gap at all by shooting, and I was getting worried it was starting to dent the rear of the receiver and create roller dimples in the walls (not good). I changed things back to original and took out the slower locking piece that I couldn't find the proper rollers for (I was trying to shoot it suppressed) and with the gap set back at it's original .35 or whatever it was from the factory, the recoil impulse felt a billion times better and it was much easier to unlock the action on. Admittedly this is a .308 roller gun I'm talking about; much less energy and violence in an MP5/clone...but the same principle exists there.
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