So I decided to build out an SP10 builders kit into a big, heavy, PRS style 6.5CM gasser. I planned on shooting the 140 Hybrids. This will be my third LFAR and haven't had any issues until now. Bought a Mullerworks 6.5 MTU blank and had a very talented smith cut it to the max size and contour possible, throated for 140 Hybrids. .936 gas block, 6lb barrel, 24" long. We just sized the gas port in his shop today firing 40gr H4350, 140's Alpha brass into a clearing barrel. We started at .080" and ended up at .085" gas port size for a +2 rifle, 24" barrel. The issue we ran into is that the 140 Hybrids are so long and pointy they fall in between the bolt lug recesses and jam into the breech. As soon as this happened I remembered reading about this being an issue. I didn't think of it before because 6.5CM 140gr Hybrid Black Hills contract ammo is shot regularly from gas guns at work. Of course those guns are built around this round and I didn't think about that. The gun ran with other more traditionally shaped bullets. We finished sizing the gas port and then went through a pile of old extensions and pulled one. We pulled the guide pin and flipped the extension upside down and started cutting a feed ramp into 2 of the other 6 lugs. We eventually found a shape, depth, and chamfer that fed 140 Hybrids without issue. Used three different mags to verify. So now we need a "blank" extension so we can cut our own feed ramp and guide pin hole. May have to run a reamer a bit deeper and re-clearance the bolt nose.
It seems like RDFs, 135 ATIPs, and 140 Hybrids generally don't feed well in gas guns because of this. I specifically ordered a 6.5mm/ 260 extension from Tritune but they removed too much material from the feed ramp in all reality for a 140 Hybrid. The bullet just drops into the bolt lug gap in the extension and spears the breach face. Other traditional shaped bullets work fine. 140 ELDMs work fine. The SOCOM 6.5 ammo doesn't work but of course it's using a 140 HYBRID as well.
So, now that we've figured out the feed ramp shape, depth, and where critical chamfers need to be, we'll figure out a blank extension. We reached out to 3 companies this evening. We'll see if someone wants to scan our modified extension and perhaps produce a Hybrid specific extension.(It will still feed any other bullet shape). But we'll settle for a "blank". We're also ordering a JP optimized extension just to try.
Working through all this, I was tempted to just say, "F'it, I'll just shoot a different bullet" but my Smith wasn't having it. I spent most of the day at his shop working on this with him. Driving home I thought about the Geisselle and Knights guns that shoot the BH contract 140gr Hybrids. They obviously have proprietary extensions that have been tuned for the Hybrid. I remember when SOCOM first started experimenting with 6.5CM uppers and waffled between 130's and 140's. The guys that pushed the development obviously stuck to the 140 Hybrid. Which in retrospect is the antithesis of a gas gun bullet in conventional wisdom. But they forced industry to deal with it. And then I thought about all the SP-10's that certain unit dumped their SR25's for. Undoubtedly they're shooting the same 140gr gasser ammo made by Black Hills. So, the Seekins SP10M's must have a compatible barrel extension....?
So, who's worked on this? Who's shooting Berger 140 Hybrids at max seating depth in a constrained mag? Who has a SP-10M and confirm or deny Hybrid compatibility? Who wants a secant bullet friendly barrel extension? (that is probably also a problem in 22 and 6mm large frames).