Whoa, hold on a minute. All standard rifle-length gas systems use triangle handguard caps. The A2 rifle handguards have a special arrangement of nubs that allow them to fit either the top or bottom of the triangle cap. Behold:
Carbine-length gas systems use round caps.
On a related point of historical trivia, the round “A2” rifle-length handguards were actually developed in the 1960s, but were not adopted for issue until the upgrade to the A2 standard in the mid 80s. More here.
Thanks for squaring me away. I must have been thinking about my carbines. Most bad the round cap and a few had a round cap with a cut out on the bottom when the bbl had an M4 profile.
thanks for the information .
so ? a standard A2 rifle’
can use either one ? but the triangular is more correct.
peabody
p.s. that was some GREAT reading ! 
them M16’s aint never going to catch on are they ?
thanks for posting that !
No. Rifle-length gas systems use triangular handguard caps. If you were to put a round cap on a rifle-length gas system, good luck finding handguards to fit it.
so what your saying is, the standard '‘round’'rifle length handguards will not fit on a round cap rifle length ?
i think i understand.
peabody.
but, im positive now’ that i’m going with the triangular handguard cap on my A2/clone build.
If this helps, look at the handguard caps and their descriptions on the BCM website:
A triangular hand guard will not fit a round cap.
http://www.midwayusa.com/viewproduct/?productnumber=881062
A triangular cap, as found on rifles and middies :D, will fit both. If you look at a around handguard you will see the groves should fit both caps. I don’t have a pair here to photo.
http://www.midwayusa.com/viewproduct/?productnumber=126536
[b]"Replacement handguard caps fit mil-spec hand guards and military profile barrels.
Notes:
Requires removal of the front sight base for installation
Will work with both A1 and A2 handguards"[/b]
Below is the type of cap I’ve had on on carbines with M4 profile bbl and M203 cut in bbl.
In the link below check out the A2 handguards and you will see a tab at 12 and 6 O’clock for the tri cap and the tabs at 10 & 2 and again at 8& 4 O’clock for the round caps.
http://www.brownells.com/.aspx/pid=5561/Product/AR_15_ROUND_HANDGUARDS#
Here is another link to a rifle with the triangle hand guards with current round handguards.
If you want to use triangular handguards get a triangular end cap, it will also work on round handguards.
Yeah what Amicus said! 
Looks like we were typing at the same time. His link is easier to look at everything is in the same place.
thanks guy’s…
didn’t mean to confuse and debate. the bravo link’ thank you.
im a littel slow…:sad: but im up to speed now !!:sarcastic:
peabody
just dug out the triangle cap, its ready to go.
Hello, Three good companies that sell 20’ barrels are: bushmaster 20"
1/9, A2 gov. profile. Green Mountain 19.5" 1/12 A1 and a 20" 1/8, A2
gov. profile and CMMG 20" A2, 1/7 gov. profile.
Merry Christmas,
Mark