PCC Sub Gun Omnibus Thread

[outrider58]

I recently built a 5" RDB 9mm upper.
1) Are there any Colt mag adapters that work w/ this? Even after removing the ejector and feed ramp of the ones I have to get the upper & lower to close, the mags are held way too high. Do I have to use Endomags?
2) Approx what weight buffer am I going to need to run? I have an 8.2oz 9mm buffer for my blowback gun. Am I going to need lighter? Shooting 147gr subs almost exclusively.

I was able to adapt the RDB to my New Frontier MP5 lower by removing material from the removable feed ramp 'wings'. Those are what stops your upper from closing. Be sure to use a standard upper receiver or one that is made for the RDB(having the cam key cutout).

This may look like a lot but it is roughly .055" shorter than the factory feed ramp on the right. It's just the angle and the shortened one is in situ while the factory one is being held there by my thumb.

 

[teratos]

I recently built a 5" RDB 9mm upper.
1) Are there any Colt mag adapters that work w/ this? Even after removing the ejector and feed ramp of the ones I have to get the upper & lower to close, the mags are held way too high. Do I have to use Endomags?
2) Approx what weight buffer am I going to need to run? I have an 8.2oz 9mm buffer for my blowback gun. Am I going to need lighter? Shooting 147gr subs almost exclusively.

You can make it work by taking a little off the top of the mag catch so the mag seats lower. I ended up putting the Scheel system in my Colt gun, and saved the RDB for a Glock lower. With the RDB, an H2 buffer seems to work fine.

 

[Growler215]

Just saw this news article about KelTec sending Ukraine $200,000 worth of Sub 2000's. Hopefully Glock can send along some fun sticks to go with them. . .

https://www.newsnationnow.com/world...-owner-sends-200k-worth-of-rifles-to-ukraine/

 

[smdub]

Thanks again all.

Yes, I've now shaved the feed ramp wings down on my magwell adapter to get it to clear the bolt. I will not modify anything in the lower (like grinding a mag catch.) That is way too much of a PITA to swap back and forth. For S&G today I machined the mag catch slot very slightly higher in one of my Colt pattern mags. That mag now latches, bolt clears, and feeds snap-caps. The cut is so small I suspect the mags will still work fine in a blowback upper (and if they don't, well, I just don't care anymore.) I'll prob add a small bit of weld to front mag stop lip to prevent from overinserting the mag and causing it to rub the bolt (like if you rested the gun on the mag.)

I'll bring some light carbine buffers w/ me and a stack of quarters to limit their travel when I go to test it.

Edit: you don't have to actually shave down the feed ramp 'wings' much. They need to be narrower more than shorter. I'll try to take a pic of what I did.

 

[Melnic]

What king of accuracy should I expect from my 8" PSA upper? I'm getting about 1- 1.5" @ 25 yards and 2.5-3" at 50 yards scoped from the bench.
I used various 9mm reloads I already had (115gr fmj, 124gr fmj and 125gr Powder coated cast)

 

[John from MD]

Sounds pretty good to me. What velocity are you loading them to?

 

[smdub]

delete

 

[Melnic]

Sounds pretty good to me. What velocity are you loading them to?

I did not chrono them but the 124/125 grains are about 1060fps in a 5" gun
I guess I can chrono them and I should keep subsonic (duh cause I'd like to go suppressed)

 

[John from MD]

The only way you may get better accuracy at that speed is if you soften up your alloy. I shoot 12 BHN at 1150 with WW 231 and can a little tighter groups probably because they are bumping up better. I have been working with some sub loads and they are less accurate than my supers.

 

[smdub]

Ok, I figured out that RDB setups don't need 9mm length buffers. Standard 5.56 buffers are fine since the bolt is the same length.

I'm getting two scratches halfway down every case. They appear to always be aligned w/ the extractor (carbon fouling shadow.) Under high magnification it appears they are being scratched towards the case mouth. Must be from the barrel extension as the ejector rotates the case into them. Doesn't hurt anything but I'll probably pull the barrel back out and hit those sharp inside edges w/ a small diamond file. Anyone else see this?

I also had to lightly touch the back of the two bottom bolt lugs w/ a file so they weren't so sharp. I couldn't pull the bolt back over a loaded mag w/ the charging handle. CMMG appears to have chamfered those two very slightly but I had to smooth it out more. The bolt lugs would catch on the case mouth lip. Everything seems to be working w/ my modified Colt mag.

 

[John from MD]

I used to get them in the beginning. They never affected the life of my cases. Overtime, the scratches diminished.

 

[teratos]

Ok, I figured out that RDB setups don't need 9mm length buffers. Standard 5.56 buffers are fine since the bolt is the same length.

I'm getting two scratches halfway down every case. They appear to always be aligned w/ the extractor (carbon fouling shadow.) Under high magnification it appears they are being scratched towards the case mouth. Must be from the barrel extension as the ejector rotates the case into them. Doesn't hurt anything but I'll probably pull the barrel back out and hit those sharp inside edges w/ a small diamond file. Anyone else see this?

I also had to lightly touch the back of the two bottom bolt lugs w/ a file so they weren't so sharp. I couldn't pull the bolt back over a loaded mag w/ the charging handle. CMMG appears to have chamfered those two very slightly but I had to smooth it out more. The bolt lugs would catch on the case mouth lip. Everything seems to be working w/ my modified Colt mag.

When I had my RDB in a Colt lower, I had to polish the lower lugs on the bolt just a little, and it ran smoother.

 

[outrider58]

Funny, it looks like the case actually rotated on whatever caused the scratch(look at the end of the scratches toward the middle of the case?). I polished my feed ramps on the barrel extension. That's probably where the damage occurred.

 

[smdub]

Funny, it looks like the case actually rotated on whatever caused the scratch(look at the end of the scratches toward the middle of the case?). I polished my feed ramps on the barrel extension. That's probably where the damage occurred.

Sorry, its harder to distinguish what is what in the pic. The cases are reloads. This marked up drawing probably helps more. In real life the new scratches are very shiny vs everything else being dull but that doesn't photograph well. If they were feed ramp scratches they'd be located under the case and not lined up w/ the extractor.

I'll polish the parts and see when I shoot it again on Sat what everything looks like.

 

[outrider58]

Sorry, its harder to distinguish what is what in the pic. The cases are reloads. This marked up drawing probably helps more. In real life the new scratches are very shiny vs everything else being dull but that doesn't photograph well. If they were feed ramp scratches they'd be located under the case and not lined up w/ the extractor.

I'll polish the parts and see when I shoot it again on Sat what everything looks like.

...and those super close-ups make things look a thousand times worse. I was just spit-balling.

 

[calicojack]

Just chamfer the two lower edges of the bolt, and use a honing stone to smooth out the bottom hump of the bolt carrier. Problem solved.

 

[smdub]

Just chamfer the two lower edges of the bolt, and use a honing stone to smooth out the bottom hump of the bolt carrier. Problem solved.

Thanks. Yeah, thats what I ended up doing to the bolt. Its interesting CMMG bothered to do a little extra machining there but a lot of folks need more. Surprising they didn't just go more aggressive ramping on the back of those two lugs.

 

[calicojack]

I polished my feed ramps on the barrel extension. That's probably where the damage occurred.

I need to do that.

 

[outrider58]

Funny, it looks like the case actually rotated on whatever caused the scratch(look at the end of the scratches toward the middle of the case?). I polished my feed ramps on the barrel extension. That's probably where the damage occurred.

I was referring to these two marks...

 

[smdub]

Those appear to me to be from the inside(back) edges of the barrel extension. There is a good ridge of displaced brass towards the case mouth under magnification at those spots. If it was rotating the brass would have gone more radially. Im guessing it whacks those lugs, bounces off a little and then gets dragged the rest of the way out making contact w/ only the two points as it sits in their valley. I stoned the back edges of the right side lugs last night so we'll see if the marks change. They were very sharp.

This looks like its going to be a fun little upper. Have a Colt mag/adapter already working and will try out Endomags soon My new handguard won't get here until next week though so it'll still be running naked this Sat. My old blowback setup was finicky w/ ammo and gassy when suppressed. I haven't used it in a long while. Really looking forward to playing w/ 9mm again.