I have had an ADCOR BEAR for many years now. I seem to have been one of the earliest adopters of the platform. Pretty much from day one had two major issues with the design of the rifle. One was the original rail system which was bulky and heavy, which ADCOR resolved by introducing a keymod rail which I had my rifle converted to with no fuss. The other was the barrel, which sported a government m4 profile, which makes sense if you are looking for a government contract like ADCOR was when the rifle was introduced but doesn't mean it still isn't a terrible profile for a barrel. So, after many years and a couple of thousand round through the gun, I finally decided it was time to change the barrel. I picked up a barrel from Green Mountain (which happened to be a contract overrun for SIG's 516 series piston guns) and got to work.

The first step was to get a barrel nut wrench from ADCOR as they use a proprietary set of nuts. A little phone tat with customer service and $65 bucks later and the wrench showed at my door.

Its a well build wrench, one side for the barrel nut, the other for rail nut. The prongs on the wrench that interface with the nuts are cut so that you can't remove or replace the nuts in the wrong order, which was help since at first I wasn't actually sure which came first.

With the generosity of my local hardware store I got the rifle locked in a vice block and went about pulling the barel.

The disassembly process was pretty straight forward and the only real issue was convincing the carbon caked gas block off the original barrel.

This was the first time I had really gotten a good unobstructed look at the gas block/key. It's worth noting that the key extends a decent distance forward of the gas port in the barrel. I guess that this would result in a functionally lengthening of the gas system to somewhere between normal carbine and a midlenth, though that just a guess on my part, no idea if that extra length has any really effect on the dynamics of the gas system.

With the barrel pulled we can see the old and new barrels next to each other. The illogical profile of the government barrel becomes rather obvious next the the Green Mountain barrel, which sits somewhere between a lightweight and a tradition medium contour in profile, which I actually really like. We also see that I am going from a 16in barrel to a 14.5 inch which will have a FOSSA-556 pinned and welded for compliance.

A quick test fit for the new barrel to make sure everything lines up properly in the receiver and we are set to finish things up.

Here is a close up of the rail and double nuts before everything is torqued down to get an idea of how the whole thing works. The rail and rail nut slide in from the rear of the barrel over the extension while the barrel nut was to come on from the front. The barrel is placed in the receiver and the rail pressed to the rear. The rail nut ( the rear most nut with the smooth outer finish ) is torqued down. Finally the barrel nut ( the one with the threaded surface visible ) which is torqued into threads inside of the rail nut. It's if nothing else and interesting departure from the standard, and fortunately neither need to be timed for the op-rod to pass through, you simply torque them to spec and carry on.

All told it was a pretty straight forward process. But it didn't end up being completely problem free. The first being that a relief cut in the original barrel was needed to create the clearance for the forward charging handle. I new this before i started, and my solution was to just remove the components for the forward charging handle. since I never really used it anyway.

The second issue was more serious. Once I got the rifle out to the range it simply refused to fully cycle, even with full power m193 and m855 spec ammo. After some troubleshooting, and ordering the gas regulator that ADCOR makes specifically for 14.5in barrels I figured out the issue.


As you can see the sizes of the gas ports on the new barrels is vastly different. The gas port on the original 16in barrel is a rather massive .090. After a 2 hour drive to the nearest gunsmith and $25 later. I had a moderately larger gas port (approximately .076) and now the rifle cycles with no complaint.

This proved to be a really fun little project, and it's nice to sort of feel like a have a little bit more ownership of the rifle now that I have taken it completely apart and changed out a major component.

Excellent work and very well documented throughout.  I wish we had more posts like this.

I'll second that. Thanks for taking the time and effort to put that together. Will you add a pic of the clearance for the forward charging handle?

I replaced a barrel on mine too, I went with a voodoo 5.45 barrel,  after I realized it was short stroking I use a taccom ultra light  polymer buffer.
Adcor BCG has an oprod attached to it, so it's pretty heavy reciprocating mass by design, no need for a 3oz std weight buffer.

I figured that moving to a lighter weight buffer would also have solved the problem. But between buying the buffer and shipping, it ended up being less expensive to just enlarge the gas port. And just on a personal note, with the ADCOR having as many proprietary parts is it does. I would rather maintain as many of the otherwise standard parts as possible.