Hi I am just getting my feet wet in the AR build/modification market but I think I have a pretty good grasp of the workings of an AR. Forgive me if I ask a really dumb question though! I have recently aquired a Colt A3 (i think.....it has fixed flattop rail) it is a sporter competition HBAR 20" barrel. I got it at a great price but what I want to do is make it a carbine length. First off is that even possible and second will any upper assembly fit the Colt lower? I have read about the big hole and little hole problem? I dont know if that applies to mine or not. Next I have seen uppers at good prices, if any upper assembly will fit (I think I know the answer to this) can I use the original BCG in the new upper assembly since i dont think most come with one? I have a lot of questions but lets just take my ignorance slow!! Thanks for any help.
AR build or Colt Frankenstein you pick!
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ugaarguy
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The upper will either drop on, or require an adapter in your lower if it has the large diameter pivot pin. You can get just about any upper to fit though.
Can you? Yes. Should you? No. The bolt will wear with the extension on the bbl. it's used with. At a minimum you should use a separate bolt with each bbl. It's easier to just get an entire BCG and leave it in the upper though.
Go slow before thinking the carbine is all that; carbine gas and short handguards on a legal 16" barrel are exactly what created some problems. The result was the industry as a whole inventing midlength - even tho there was no military application at all.
The little thing consumer shooters miss is that the issue M4 uses a 14.5" barrel. That's a NFA item to you or me, with BATF stamps and paperwork to possess. Gas length isn't a bunch of different sizes like everyone thinks - it's ALL about 5-7 inches from the MUZZLE, which gives the optimum timing and best reliability.
Moving the gas port an inch and a half earlier works for some guns ok, but in the big picture, opens the bolt too soon, and speeds up the carrier to a higher cyclic rate, which has been a issue from day one. The rifle gas system was never intended to handle it.
Cutting down the HBAR barrel won't help at all, it would be rifle gas on an overweight carbine barrel. The gas port timing would be late, with reduced power because the bullet leaves the barrel and dumps it too soon. It needs that 5-7 inches, which is why the guys building rifle length sights on 16" barrels use a low profile gas block at midlength under the handguards, and the FSB out at rifle. You get the correct operation and long handguards to reach further out on the protected barrel to shoot.
PSA and others offer drop on uppers for $400, converting that 20" HBAR to 16" midlength. Another factor is that a 4" shorter barrel loses about 50fps per inch in 5.56, and the loss of about 200fps does shorten the range and reduce the terminal effects. It's the heart of the M4 debate about whether it hits hard enough in combat, being 5.5" shorter in military issue. It's also why alternate calibers like 6.8SPC were created, to boost power back up 40% out of 14.5 to 16" barrels.
Since the AR doesn't come with a fancy adjustable gas block, and relies on port size and location, plus a one specification ammo load to achieve unfailing use in the field, it's best to use the proper combinations in shorter barrels that guarantee results. And that means low powered cheap import ammo can malfunction.
The little thing consumer shooters miss is that the issue M4 uses a 14.5" barrel. That's a NFA item to you or me, with BATF stamps and paperwork to possess. Gas length isn't a bunch of different sizes like everyone thinks - it's ALL about 5-7 inches from the MUZZLE, which gives the optimum timing and best reliability.
Moving the gas port an inch and a half earlier works for some guns ok, but in the big picture, opens the bolt too soon, and speeds up the carrier to a higher cyclic rate, which has been a issue from day one. The rifle gas system was never intended to handle it.
Cutting down the HBAR barrel won't help at all, it would be rifle gas on an overweight carbine barrel. The gas port timing would be late, with reduced power because the bullet leaves the barrel and dumps it too soon. It needs that 5-7 inches, which is why the guys building rifle length sights on 16" barrels use a low profile gas block at midlength under the handguards, and the FSB out at rifle. You get the correct operation and long handguards to reach further out on the protected barrel to shoot.
PSA and others offer drop on uppers for $400, converting that 20" HBAR to 16" midlength. Another factor is that a 4" shorter barrel loses about 50fps per inch in 5.56, and the loss of about 200fps does shorten the range and reduce the terminal effects. It's the heart of the M4 debate about whether it hits hard enough in combat, being 5.5" shorter in military issue. It's also why alternate calibers like 6.8SPC were created, to boost power back up 40% out of 14.5 to 16" barrels.
Since the AR doesn't come with a fancy adjustable gas block, and relies on port size and location, plus a one specification ammo load to achieve unfailing use in the field, it's best to use the proper combinations in shorter barrels that guarantee results. And that means low powered cheap import ammo can malfunction.
JustinJ
Member
Yes, you can convert it to any configuration you like but i wouldn't. Just build a different upper. Then you'll essentially have two guns. Colt also has a reputation for over torquing their stuff as i can attest to from when i had to remove my 6720 barrel nut to install a free float rail.
Cosmoline
Member
I can sure second that over-torquing issue! I had to take my Colt Gov't Carbine upper to a smith to get the drift pins out, and ruined a really nice punch trying it myself.
JustinJ
Member
If you're talking about the front site base, that is a common with many ARs. The tapered pins flatten out when hit with a standard punch. I had to cut a FSB off one after making that mistake. Just use a concave tipped punch and they'll come out easy. Worked like a charm on my Colt.
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