Converting an old Bushmaster M4 carbine to .300 BK

D

Deaf Smith

New member
So, what do I need to do?

My M4 has the fixed carrying handle and M4 contour barrel. It shoots great but I'm thinking of a 'super' M1 Carbine concept. Just keep the upper but change barrel? Is that all that is needed?

Cost?

Or just get the whole upper and go flattop?

Thanks,

Deaf
 
R

rickyrick

New member
It's easy, barrel, gas tube, gas block, whatever hand guard system you want.

I'd replace the bolt with the new barrel, sounds like it's a bit old, don't want headspace issues.
 
R

rickyrick

New member
I'll add that flattop uppers can be had cheap and just build a new upper for 40-100$ more.

You could potentially sell your carry handle upper, some people are looking for those.
 
S

smee78

New member
While it is easy to convert your old upper I would not. Keep it and either buy or build your 300BLK upper. I would use a flat top upper and go from there, if you build you will need a few tools for the build.
 
M

Mobuck

Moderator
"While it is easy to convert your old upper I would not. Keep it and either buy or build your 300BLK upper. I would use a flat top upper and go from there, if you build you will need a few tools for the build."

I agree that trashing your present upper for the 300 experiment would be illogical. The cost savings would be insignificant.
 
M

mavracer

New member
What do you plan on using for sights?
There's a few ways to go on a front sight gas block combo.
To be honest I'd echo the build a complete upper idea.
 
R

rickyrick

New member
Being that it's probably a carbine or a middy gas system, and many 300aac barrels are pistol gas, you may have a tough time getting your hand guards to work. There are carbine gassed 300aac barrels out there, but, I don't know the drawbacks. I've emailed companies about why pistol gas on a 16" inch barrel and the answer was "more reliable". I haven't enough experience with the 300aac to know myself.
 
D

Deaf Smith

New member
Since this is a more-or-less stock M4 configuration with fixed carrying handle and all I added a fixed buttstock, then I want to keep it the same. Just swap barrels or whatever is needed and use as-is. I simply set the sight for 150 yards and leave it alone! Probably 125 gr. loads.

Now I have a DPMS 3G2 16 inch barrel competition AR for scope work and thus I want to keep this one as simple as possible. Would love if the barrel was actually at 14 incher and the last two inches a fixed flash hider (I don't know if they even make that) so it becomes a legal 16 inch bbl.

I shoot aperture sighted rifles often so I am well versed on iron sights.

Deaf
 
D

damienph

New member
If your older Bushmaster is a 16" A2 style it is probably a government profile barrel which means it has a barrel diameter of .750" under the gas block front sight base (FSB). The lightweight barrel contour has a barrel diameter of .625" under the FSB.

It also most probably has a carbine length gas system, my 16" Bushmaster A2 does.

16" .300 AAC BLK barrels with a carbine length gas system are not as common as the pistol length gas system but they are readily available. Find a 16", .750" carbine length gas system barrel and all of your other parts will work. The only issue is the new barrel will probably not be drilled for the FSB taper pins (not an easy task for the home gunsmith and about a $90 bill to have a real gunsmith do it for you). There are a few companies that make an FSB with set screws in place of the taper pins but I have found that they are usually "out of stock".

My 16" carbine length .750" barrel came from SOTA Arms.

http://sotaarms.com/component/jshopping/product/view/13/113?Itemid=0

I did use a flattop upper (Aero Precision), a YHM low profile gas block and tube, an Aero 15" free float Keymod tube, and a SOTA A2 style .30 cal flash hider.

Didn't cost an unreasonable amount of money and it is very accurate.
 
I

Ibmikey

New member
The basic thing overlooked here is subsonic or supersonic? What do you want to do with your blackout? My pistol has an 8.5" barrel, pistol gas and a Faxon Loud mouth brake...a real fun supersonic shooter, while i built a 16" barrel carbine with a light weight barrel and carbine gas, again for supersonic shooting. Both weapons perform flawlessly with the ammo intended for them, i tried some subsonic ammo a friend used with his pistol gas carbine and a can....worked for him but i had a single shot as it did ńot function adequately with carbine gas system.
 
B

Bartholomew Roberts

Moderator
In theory, all you need is a new barrel (and a drill press and solid machining skills). In practice, you nave a pinned, fixed front gas block. I suppose in theory, it could be swapped to a new barrel but I've never known anybody desperate enough to want to try to drill matching taper pin holes in a new barrel in order to save $26.

You could also reuse the carbine length gas tube; but the .300 works best with pistol length gas. So low-cost solution is a new .300 pistol-gas barrel, pistol gas tube, and frontsight/gas block for around $130 (for everything). You can pick up brand new assembled flattop .300 uppers (no BCG or charging handle) for as low as $210. If you whip out the drill press and reuse everything you can, you can get it down to about $90 for a new barrel.
 
S

smee78

New member
Well now you have to add the cost of proper tools to "rebuild" the upper if you don't have them already.
 
You must log in or register to reply here.
Top