I am looking to finally have my AF style carbine rebuilt, this time with an M4 style barrel to replicate a GUU-5/P

Did the AF carbines that were rebuilt (with the newer M4 profile barrels) use the standard round carbine handguard cap, or the later notched style handguard caps?

I know there are a lot of variables with these AF rebuilds, but the M4 type barrels should have been received by the AF with the handguard caps already installed.  For this reason I would imagine it was a standard part, whichever variation was used.

I don't know for sure, but here is my thinking on this.  The step cuts in the cap are for the M203.  So if you are using a barrel with a M203 cut you use the cap with the M203 steps.  For a pencil barrel carbine you would use the round cap without the steps.

When I see a pencil barrel with a M203 cap it looks wrong - no logical reason they should have been built that way.  Same thing with a M203 profile barrel and a round cap - it wouldn't have been built that way.

I could be totally wrong, but that is how I see it.

My uneducated guess with the GUU-5/P carbines is that they are basically modern M4 barrels slapped on the older GAU-5/P lowers and the FCGs modded to replace the auto setting with bust packs.



A close up of the GUU-5/P here shows that the barrel has the M203 cut handguard cap and barrel of an M4, but pretty much retain everything else from the earlier GAUs, including the flat handguard ring, slickside upper, partial fence lower, 6 hole slim handguards and even the old 2 position stock of the old XM-177 carbines from Vietnam.

This will definitely help out in getting your GUU-5/P built.

Can't a 203 be slapped onto a pencil barrel?  I thought the M203 barrel profile was an A2 barrel with a step cut back to .625 to fit the launcher, and the clearance cuts on the cap are used for allowing the 203 to be slid forward for loading?

It could. It was rarely done though since a device (usually a tube around the barrel held on by the flash suppressor) has to be put on the barrel to keep the M203 from moving forward. If this was done it would be a custom job, and so the armorer could also cut clearances into the handguard cap by hand while they were at it.
Dustin

When it comes to barrels, they come as a unit where the front sight post, sling swivel and handguard cap come together already pre-installed.  The GUUs I have seen that have M4 barrels had the cap with the notches and the pencil barrels have the cap without the notches.  Can't say I ever seen it done any other way.  As far as the "go fast" parts go, every GUU I've ran across was full auto but never seen them with the burst trigger group.  Again, in the Air Force, most anything goes but if there's a GUU out there different than what I've seen and described, that would be a new one to me.

Thanks for the replies and info.

I thought it used the notched cap to match the 203 mounting capability of the barrel, but couldn't adequately zoom on any of the GUU pics on RBR or elsewhere on the net to confirm this was the case.

For my clone I'm using a 1/7 Bushmaster 14.5" barrel on my 604 upper (along with a Denny's Guns extended A2 and a peel washer).  

The barrel's FSB is not F-marked, and has the 203 notched handguard cap (both which I now think would be correct features for the AF rebuild barrels).  Unfortunately being a newer production Bushmaster barrel, it has an M4 barrel extension.  Not that it will matter using it with my non-M4 604 upper, I would have just preferred it not have the extended feedramps as they unnecessary with this upper.

It will be cool to get this carbine put back together.  It used to be a 14.7" lightweight GUU clone on a 1/7 BM superlight, but for some reason I always wanted to do one with the M4 type barrel, although now I'm sure I'll be asked not to come back to the retro forum for such poor judgment

heres mine, just need to put my partial fence lower on

Very nice indeed.  Makes me anxious to get mine back together.

I'm using a full fence NDS lower, a no tic-mark selector, and a Colt 2-pos extension with old style nut.  Unfortunately I've never dropped the money on a coated aluminum stock, but I figured since I'm doing a later style conversion the N marked stock would be just as appropriate.

It's pretty tempting to rattle can this thing CCT/Pararescue style when it's done, but I'm still undecided on that.  First step will be putting some rounds through it with the new barrel

I just found this pic indicating that the notched cap is correct for this barrel upgrade: Pic

GUU-5/P's are NOT burst conversions.

S/F

Al

I carry a GUU-5 daily and mine has the notched style cap, M4 barrel, A2 upper, and a 4 position tube with current style M4 stock.
SAFE/FIRE/AUTO

We've got over 100 of 'em, and they're all the same.

There should be two basic styles of GUU-5/P from what I've seen.  The first are those simply using the M4 barrel assembly on existing weapons.  The others involved the use of complete 727-style fixed carry handle uppers.  Both are now slated to be replaced by stock M4A1s in the USAF.

Hell, I like it just the way it is! I like the slabside with M4 barrel. Also like C7 uppers w/M4 barrel.

Regular M4 barrels and these GUU replacement barrels have different stock numbers and the GUU barrel is quite a bit more expensive. What it comes down to is F marked FSB's.

What about feedramps?  Do the GUU replacements have standard, rifle ramps since they're going on non-M4 cut uppers, or do they have M4 type ramps just like the M4 production barrels?

The only thing I've ever seen that is the same from one GUU-5/P to the next GUU-5/P is them having the old slip ring and not a delta ring. Seems like everything else changes from gun to gun; the armorers were assembling parts as they had them and in no particular order... they both have the same NSN.


We don't have any of em around these parts with an A2 upper; I've only ever seen some smaller pics of them on the interweb. If you have a chance, could you please post some nice pics of em?

…and for the OP's question

Great pic, thanks!

Can you tell us what type of feedramps these barrels have (extended or standard extensions)?

Doesn't change the fact that there are two basic styles, one where the entire upper receiver was replaced and ones where the barrel only was replaced.  As far as the USAF is concerned, I get the feeling that NSN essentially just applies to 5.56mm carbines that aren't GAU-5A or GAU-5A/A.  The NSN applies to the original 14.5" A1 type rebuilds (GAU-5/P) as well.

Really, NSNs reflect the paper configuration of the weapon, for supply purposes, so that when you order something you get something equivalent.  A good example is the M16A2.  All are considered equal in the eyes of the supply system, whether they came from Colt, from FN, or were rebuilt from A1s.  They all have the same NSN and are all M16A2s, despite the configurations not being identical.  Even when a MIL-STD is modified, the nomenclature and NSN don't necessarily change.  The recent changes to the M16A4 MIL-STD are a good example of this.

I see what you're saying. Didn't understand as I've not seen a GUU-5/P with a complete new top end; the only ones I've seen are "rebuilds."