...the differences between an M4, Dissipator, and standard 16" barreled uppers?  I know its a pretty rookie question, but I hear them all the time and am begining to get confused.  If I have it right, an M4 has the short barrel, short, fat handgaurds and a flat-top upper.  The Dissipator has a short barrel, full length handgaurds, carry handle.  The standard 16" version would be 16", short regular handgaurds and a carry handle.  How close is this?

I have a BM Dissipator.  It has a 16" barrel and full length handguard.  It is a great rifle with one major defect.  Instead of just shortening a 20" barrel, they used a 16" carbine barrel with the gas block mounted to it in the short handguard location.  Then chopped off the front sight riser and mounted another gas block and sight riser at the end of the full length handguard.  This adds extra weight to the front of the rifle.  Bad choice in my opinion.  Otherwise great shooter.

The M4 has a government profile barrel under the short handguard and a reduced diameter barrel in front of the front sight for mounting a grenade launcher.

Ths standard 16" has a short handguard with a 16" heavy barrel.

You have pretty much got it all correct. A true M4 has a 14.5 inch barrel with the step cuts in the barrel for attaching an M203 grenade launcher. These models also have the fat handguards usually but some makers such as Bushmaster don't use the fat handguards on their commercial M4 designs. The Bushmaster M4's I am refering to are the post-ban models with the muzzlebreak attached to make them legal. The Dissipator has a 16" barrel with full length handguards and is designed to be a more compact model but retain the sight radius of the 20" models. The 16" carbines are usually designed to be more compact and lightweight than the the 20" models.

Thanks m193, I was pretty sure I knew at least close to what the were.  Appreciate you clearing that up for me.

So am I right in assuming that the M4 uppers on the market have 14.5" barrels and they use a longer muzzle break or flash suppressor to make them the legal 16"?

Quoted: So am I right in assuming that the M4 uppers on the market have 14.5" barrels and they use a longer muzzle break or flash suppressor to make them the legal 16"?

Yes and no. M4 is a term that gets tossed around loosely on the boards and means lots of different things. To answer the first part of your question. A true M4 barrel (military spec) is a 14.5" 1:7 twist barrel with the step cut. Commercial models must have a permanently attatched flash hider. Some companies also offer an "M4 Style" upper with the step cut but on a 16" barrel. These will not fit a bayonet though as they are too long between lug and flash hider. Then there are post ban M4 style with the step cut and a muzzle break. Again... some are 14.5" and some are 16". Take a look at the Bushmaster catalog and you will see.

Now for the info on a TRUE Military Spec M4:

- 14.5" 1:7 twist barrel with step cut.
  - Underneath the handguards are several markings on the barrel (on Colt M4's). One is a very faint "C" near the foward end of the handguards and the other is a "4" that is near where the barrel meets the reveiver (if I recall correctly).
- The military has used both A2 and Flat-top models of the M4 although most are now flat-tops. If it is a flat-top, factory assembled M4 then the front sight base will be marked with an "F". They are different than the standard front sight by about .030". I don't know why.
- The military spec M4's (and most commercial models) use a fatter handguard with double heat shields.
- Finally the upper receiver itself has some differences. A true M4 receiver has a "4" stamped above the hole for the gas tube on the front end. Then the receiver has two feed ramps that are cut into them that mate up with the barrel extension on an M4 barrel (see www.ar15.com/forums/topic.html?id=118733)
- And lets not forget a "True" M4 has a "Fun Position" on the selector switch. Some older military M4's were Safe-Semi-3RB but the current spec is for Safe-Semi-Full Auto (correct me if I'm wrong on this)

So to recap, while most people (I myself am guilty) toss around the "M4" term in respect to different features on their rifle (most commonly referring to the barrel profile or handguards). The M4 is the military designator for a carbine length M-16 with specific features and specs.

And, again. Please don't flame me but politely correct any mistakes I've made. This info is coming from a somewhat hazy memory after talking with a friend who is a Certified Colt Armorer as we detail stripped a Colt M4 in a hotel room at the last Oregon machine gun shoot.

Hey they are, in pictorial form:

Bushy M4:

Barrel Assembly:
Preban:

Postban with Mini Y Comp

Post/Preban with AK brake

Postban full 16 inch barrel


Bushy Dissapator:

Barrel Assembies
Hbar (origional)

M4 (lightweight)


Bushy A2 Shorty Carbine (Hbar)

Barrel Assembly

personally i dont care for the disipator.  it looks ugly to me.  but thats ok, cuz i plan to buy a M4 next year.

Quoted: personally i dont care for the disipator.  it looks ugly to me.  but thats ok, cuz i plan to buy a M4 next year.

They look a little better when you have a muzzle break or pre-ban form with a flash hider and bayolug. I think it gives them a little bit of a short barreled rifle look. But I'll stick with my M4's for now.

Quoted: I have a BM Dissipator.  It has a 16" barrel and full length handguard.  It is a great rifle with one major defect.  Instead of just shortening a 20" barrel, they used a 16" carbine barrel with the gas block mounted to it in the short handguard location.  Then chopped off the front sight riser and mounted another gas block and sight riser at the end of the full length handguard.  This adds extra weight to the front of the rifle.  Bad choice in my opinion.  Otherwise great shooter.

Not exactly any other way to do it and still be reliable function wise.