PCC Sub Gun Omnibus Thread

The #1 community for Gun Owners of the Northeast

Member Benefits:

  • No ad networks!
  • Discuss all aspects of firearm ownership
  • Discuss anti-gun legislation
  • Buy, sell, and trade in the classified section
  • Chat with Local gun shops, ranges, trainers & other businesses
  • Discover free outdoor shooting areas
  • View up to date on firearm-related events
  • Share photos & video with other members
  • ...and so much more!
  • outrider58

    Pronouns: It, That
    MDS Supporter
    Jul 29, 2014
    39,626
    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.
     

    Attachments

    • feed.jpg
      feed.jpg
      37.4 KB · Views: 165

    teratos

    My hair is amazing
    MDS Supporter
    Jan 22, 2009
    51,666
    Bel Air
    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.
     

    smdub

    Active Member
    MDS Supporter
    Nov 14, 2012
    3,855
    MoCo
    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

    Active Member
    MDS Supporter
    Dec 27, 2012
    13,654
    HoCo
    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)
     

    Melnic

    Active Member
    MDS Supporter
    Dec 27, 2012
    13,654
    HoCo
    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

    American Patriot
    MDS Supporter
    May 12, 2005
    17,841
    Socialist State of Maryland
    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

    Active Member
    MDS Supporter
    Nov 14, 2012
    3,855
    MoCo
    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.
     

    Attachments

    • 20220322_extension_scratch.jpg
      20220322_extension_scratch.jpg
      61.2 KB · Views: 109

    teratos

    My hair is amazing
    MDS Supporter
    Jan 22, 2009
    51,666
    Bel Air
    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

    Pronouns: It, That
    MDS Supporter
    Jul 29, 2014
    39,626
    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

    Active Member
    MDS Supporter
    Nov 14, 2012
    3,855
    MoCo
    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.
     

    Attachments

    • 20220322_extension_scratch_markup.jpg
      20220322_extension_scratch_markup.jpg
      65.2 KB · Views: 123

    outrider58

    Pronouns: It, That
    MDS Supporter
    Jul 29, 2014
    39,626
    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

    American Sporting Rifle
    MDS Supporter
    May 29, 2018
    4,003
    East Bumfkgypt
    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.
     

    Attachments

    • IMG_1846.jpg
      IMG_1846.jpg
      86.9 KB · Views: 112

    smdub

    Active Member
    MDS Supporter
    Nov 14, 2012
    3,855
    MoCo
    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.
     

    outrider58

    Pronouns: It, That
    MDS Supporter
    Jul 29, 2014
    39,626
    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...
     

    Attachments

    • extension scratches_LI.jpg
      extension scratches_LI.jpg
      58.4 KB · Views: 83

    smdub

    Active Member
    MDS Supporter
    Nov 14, 2012
    3,855
    MoCo
    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.
     

    Users who are viewing this thread

    Latest posts

    Forum statistics

    Threads
    262,854
    Messages
    6,728,433
    Members
    30,820
    Latest member
    82peterbilt

    Latest threads

    Top Bottom