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Posted: 12/25/2025 7:10:49 AM EDT
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I’m considering making an AR PCC in 357 Sig. But, cutting a longer throat so bullets can be loaded out, taking advantage of the extra length of the Glock mags I use, and my 1911. That gives extra room in the case for a little extra powder. I know that a bottleneck cartridge and a blowback can be problematic. But, I think that using a 10mm buffer I can make it work. What say the hive? https://imgur.com/a/XRJGkt1 |
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Originally Posted By Sailormilan2: I’m considering making an AR PCC in 357 Sig. But, cutting a longer throat so bullets can be loaded out, taking advantage of the extra length of the Glock mags I use, and my 1911. That gives extra room in the case for a little extra powder. I know that a bottleneck cartridge and a blowback can be problematic. But, I think that using a 10mm buffer I can make it work. What say the hive? https://imgur.com/a/XRJGkt1 1st....take some time to learn how to use the toolbar that is located above the message box. Those tools allow you to post pics, or make your link hot. 2nd....I would contact Macon Armory (Rudy) and see if he can ream your barrel chamber for 357 SIG, or if he has done it before and see what his results were if he has done it. 3rd...The base rim diameter of the .357 SIG is like .420 I guess you could use the 10mm RDB system, which would be the path I would go down. A .40 S&W BCG could also work. I think it will be a wash considering you can build a 10mm and use off the shelf components including a 10mm RDB system from CMMG. 10mm ammo is also cheaper to buy or reload. But if you have to have the higher velocity of a .357 SIG, and you want to deal with building it.....do it. Because it will be a very unique PCC Here's the pic you tried to load..... 
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Occid Vel Occisus Est !!
| CMMG finally released the FE (Fixed Ejector) Retrofit kits in February 2025 to resolve all the ejector spring failures in 9mm and 45. They said they would be coming out with a kit in 10mm I would wait for that. 40SW and 10mm both eat the ejector springs with the CMMG RDB system |
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I think you could do it. I knew nothing about building guns until meeting the people on this particular board. I made something kind of similar. Its a gas piston 9x25 Dillon. I'm still trying to iron out some of the kinks(case head separations every once in a while) and I need to have a stronger op rod made as my titanium one is beginning to take a bend. If you're interested I posted a youtube short below. https://www.youtube.com/shorts/ww9bkNA0bGc |
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Originally Posted By tawoodii: I think you could do it. I knew nothing about building guns until meeting the people on this particular board. I made something kind of similar. Its a gas piston 9x25 Dillon. I'm still trying to iron out some of the kinks(case head separations every once in a while) and I need to have a stronger op rod made as my titanium one is beginning to take a bend. If you're interested I posted a youtube short below. https://www.youtube.com/shorts/ww9bkNA0bGc I’ve got a 9mm barrel for both a 1911 and an AR that I can convert. Everything is on hold until I fix my lathe. It needs a new motor. I’ve looked at the 9x25 Dillon, and I think this will come really close to matching the Dillon round. The bullets won’t be seated as deep in a case taking up powder volume. It also allows for a wider range of bullet styles. The cast lead bullet pictured, a Saeco 383, should work in an unmodified gun. The other ones require that the throat be modified by lengthening. But not changing the headspace allows standard factory loads to be used. |
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Originally Posted By tawoodii: I think you could do it. I knew nothing about building guns until meeting the people on this particular board. I made something kind of similar. Its a gas piston 9x25 Dillon. I'm still trying to iron out some of the kinks(case head separations every once in a while) and I need to have a stronger op rod made as my titanium one is beginning to take a bend. If you're interested I posted a youtube short below. https://www.youtube.com/shorts/ww9bkNA0bGc Are you using Starline brass? The only handgun case head separation I've ever experienced was in 9x25 Dillon converted Starline 10mm brass. |
| Yep I am pretty sure it is. I've been waiting on Dillon dies 9x25/357sig to begin reloading my own. I purchased the 9x25 ammo from Underwood and I believe they source their brass from Starline. I had the headspace checked by the barrel maker and it came back fine. One in 10 of my fired cases either suffers separation or has the little gold ring of death upon inspection. |
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I have built a few over the years, and you probably DON,T want to do a blowback in .357 sig.....even with an 11 ounce deadblow buffer , OOB was still a regular occurance and they are basically unpleasant to shoot. I remember BRUTAL trigger slap in the prototype guns. I still have one 16 inch barrel , but it is not mounted in an upper. Been thinking about doing a DI for an Air Marshall bud, but finding a Glock mag lower to work with DI or RDB for that matter could be an issue. Small frame glock lowers are supposed to be for blowback actions....but with so many lowers with the mag height wrong on the market, might not be as hard as I I imagine. Rudy |
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Thanks for the reply. Right now the project is on the back burner until I get my lathe fixed. But, IMHO, an 11oz buffer is too light. By my calculations, I’ve managed to get the bolt weight up to about 16oz. Which means I need a buffer weight of at least 14oz. Or, a combined weight of about 30oz. For my 10mm I’ve got it functional at 35oz(combined weight). But, that’s with a “jury rigged” buffer weight that has 3 parts. 5.6oz(homemade tungsten buffer weight) + 2.5oz(buffer weight/spacer) + and 10.8oz KAK buffer (advertised as 11oz). With a functioning lathe, I can clean that up by making a modified rifle style buffer out of steel, instead of aluminum, and tungsten buffer weights. That should work well for a 357 Sig. it might be a little more difficult to a weight for a Carbine stock. But, I think that a combined weight of what is used for a 10mm would work. Hopefully I’ll know soon. |
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You could always reach out to Ron Williams, he does DI pistol caliber uppers. One of his .357 magnum ones is on my bucket list. He already does a 357 Sig upper, so he might just be willing to modify the chamber by request. https://rmwxtreme.com/357-sig-di/ |
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You’re going to have setback from loading long if you use jacketed bullets, probably plated, too. Might be able to get away with it with coated bullets. Not sure. I’ve often thought about getting a .40 RDB bolt and asking a gunsmith to rechamber a 9mm RDB barrel, but then when I consider cost and utility, I remember that 5.56, BLK, .338 ARC, etc already exist. But I do think .357 SIG would have been an awesome supersonic SMG cartridge if there was mainstream support, like if Sig had ever made the MPX that way. |
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