.22 Colt IMP

I am starting a new thread with this request, after reading a previous but current thread.

Is there such a thing as the .22 IMP cartridge, or is it just a slang term for the .221 Fireball? I am talking in regard to military testing of a lightweight weapon. If so, could someone give us a brief history of it?

I find I have nothing in my files on this round. I don’t even know what IMP stands for, if anything. Can someone tell me?

Regarding the weapon made, which I assume was the Colt Lightweight SMG, does anyone remember what company made an awful semi-auto pistol (SMG look-alike) in .223, based on this Colt gun? It was not Colt, that I recall. We had one go thru our shop and took it out for test firing and to see what it was about. I remember one section of the weapon swiveled, but I forget the context of why it did. It was ghastly. Inaccurate, undependable, difficult to shoot even with the poor parameters of accuracy it had.

Anything anyone can tell me about the .22 IMP cartridge, or this commercial firearm - two separate, but related subjects due to the latter being a copy of the Colt gun, in concept anyway, will be appreciated.


IMP is Individual Multi-Purpose Weapon. GUU-4/P is the pistol you are thinking of, I think?

I have been studying the IMP for a day and a half now and have become quite the exspurt. ;) ;)

The link that Daniel Watters posted is an excellent start.


JOURNAL #254 has a very good article on the cartridges.

There was a thread on the Forum some time back that also discussed it.

iaaforum.org/forum2/viewtopic.ph … hlight=imp

Anyone interested should start with these three references.


The commercial copy in .223 was the Bushmaster Arm Pistol. It was designed by Mack Gwinn Sr. and Jr. The younger Gwinn was an US Army Special Forces officer during Vietnam and had encountered the Colt IMP somewhere along his career. The pistol grip swiveled so that the body of the weapon could lay against the forearm. The forearm would then support the weight of the weapon and also soak up some of the recoil.

I hope that the younger Gwinn was a better Special Forces Officer than he was a gun designer, because I can tell you from personal experience that the gun was a total piece of crap. We actually would not sell it. I think we sent it off to the gunsmith we did business with to break it up for parts, and just took the loss on it. We couldn’t really send it back, since we had taken it out and fired it, and it got a little scuffed here and there around the ejection port trying to clear horrific jams.Plus, past 15 yards, we couldn’t hit diddly squat with it. It may be one of the worst firearms I have ever shot in almost 60 years of shooting. If the military version was no better, it is no wonder it went no where.

You are right though, it was called the Bushmaster, as I recall now. thanks for the info. Wanting knowledge about arms and ammunition is not contingent on them having been any good.

From reading the source material referenced, and the answers, I am led to believe that there is no such thing as the .22 IMP cartridge, but rather simply military loadings of the .221 Fireball for the Colt IMP firearm.

.22 IMP as a name for a cartridge seems to be a misnomer.

Am I making fair statements there, or am I missing something?

Having asked to be told of the IMP round, I find I have both the ball and the tracer in my collection, and while I don’t know where they originated from, I didn’t get them from Lew. They might have come from him originally though. I have them identified simply as the .221 fireball loaded for a Colt SMG.

Are you saying then that .22 Colt Imp cartridge in your collection is identical to the .222 High Velocity (headstamped R - P) referred to in Ray’s SCHV thread - or am I falling behind again?

Jim - I am not quite sure what you are reading??? I didn’t say a word about my cartridge being identical to any round from Ray’s article. I don’t collect those rounds and haven’t any to compare with them, so it would be impossible for me to make the statement you are implying. He didn’t picture any round with an R-P headstamp for me to compare mine to, either, unless I missed smething.

My cartridge (I can only relate to the ball round - the tracer was another victim to California law and is long gone, although still in my catalog), but my cartridge is headstamped “R - P 221 REM” and as far as I know, is identical in case dimensions to the .221 Fireball. Case length is 1.393"; rim (head) is 0.3745"; base is 0.3725"; dimension just below shoulder is 0.357"; neck is 0.249". Unfortunately, I don’t have a civilian .221 Fireball in my dupes to compre it with. I don’t collect that cartridge normally, since it is neither for an auto pisto nor an SMG.

The primer is copper-cupped, and therefore probalby a 7-1/2 Remington factory primer. The bullet measures 0.221" at the case mouth, probably larger in diameter inside the case mouth (0.224" ???) and is a FMJ spitzer.

All I actually said, and it was more a question, was that it seems to me there is no such thing as the .22 IMP cartridge, but rather simply the military loadings of the .221 fireball to fit an IMP firearm. Those are two quite different things. One can call a .30-06 a .30 Garand cartridge if he wants to, but the fact is, it is still a .30-06 for which the Garand is chambered.

I then asked for confirmation of that belief.

Nothing was said at all about any comparison to any cartridge Ray wrote about.


John can answer for his cartridges but after this IMP thing started I dug around through my cigar boxes and found one that I had. It is a 221 Remington Fireball loaded with a FMJ bullet. I believe it’s one of the un-painted Tracer cartridges that Lew has. In comparing it with the description in the JOURNAL article, it matches exactly right down to the copper primer.

Since the 221 Fireball was a pistol cartridge I had tossed all of my dupes into a cigar box of other pistol rounds which is probably why I had overlooked it for years.

You need to get the three references that I listed. After you read them I think you’ll come to the same conclusion as John and I. The IMP cartridge is a 221 Remington Fireball.

Exactly what that 222 High Velocity round is we have yet to determine. The same for the CAL 22 CARBINE with the sp bullet and FA 71 headstamp.