A lot of people consider 12.5″ to be the best barrel length for a general purpose AR-15, as they believe it provides the best balance between compactness and ballistic performance. For our new build, I wanted a handy carbine, suppressed and unsuppressed, and I wanted to be able to get hits at longer distances as well so a 12.5″ AR-15 sounded perfect. I looked into several options for barrels and ended up settling on one from Centurion Arms and I’m glad I did.
My priorities for a 12.5″ barrel were accuracy, weight, reliability, and barrel life. A stainless barrel might be the best option for accuracy but they tend to be on the heavier side and have tighter chambers. I wanted a 5.56 chamber for maximum reliability under all conditions. Chrome-lined barrels supposedly have a longer barrel life than both stainless and non-lined barrels so I tried to find a lighter weight 12.5″ chrome-lined 5.56 barrel and came across Noveske and Centurion Arms’ hammer forged barrels. These two options intrigued me since they both have a great reputation for accuracy. Their 12.5″ lightweight barrels look identical as they both have a slight taper in front of the gas block on what otherwise looks like a gov’t profile barrel. I also believe they’re both made by FN to their specs. Although we’ve had two Noveske uppers in the past and they both shot well, I decided on the Centurion Arms since we’ve never owned one before and wanted to see if their tapered bore made a difference.
Centurion Arms uses a tapered bore, which means that the bore gets slightly smaller as you go down towards the muzzle, which is supposed to help with velocity and perhaps accuracy. There is a technical explanation of the process and the effects from the owner of Centurion Arms, Monty Leclair, here. I wasn’t sure how much of a difference it would realistically make, but since we also have an 11.5″ Daniel Defense build I could at least compare the velocity numbers to it and see what kind of speed increase the Centurion Arms 12.5″ gets with an extra inch of barrel.
For the rest of the build, I went pretty vanilla. I used a 10″ Daniel Defense Lite handguard. I’ve used free-float tubes in the past and came back to quad rails because I don’t mind the grip and appreciate the rigidity and versatile mounting options. The barrel, which came from Centurion Arms dimpled for a set screw gas block, was installed on an Aero Precision upper receiver with a BCM BCG and a Gunfighter Mod 4 charging handle. The muzzle device is a SilencerCo ASR flash hider for our pending Specwar K. The lower has a Geissele SSA trigger and Magpul CTR and MOE grip. The BUIS are KAC Micro 2-600m, which I feel are the best BUIS since they’re easily windage and elevation adjustable, and provide a great sight picture. The optic used was a Burris XTR II 1-5x with 5.56 Gen 3 reticle (you can read a quick review on it here) in a Larue SPR mount. With no bipod and a 30 round D&H magazine loaded with 77gr ammo, it weighs 9.0 pounds. With a bipod and 20 round NHMTG magazine loaded with 77s, it weighs 9.8 pounds.
One concern I had about the barrel prior to firing was the smaller .063″ gas port. I read a couple posts about bolts not locking back on empty magazines, failures to cycle, etc. but our rifle has been 100% reliable with a standard carbine buffer through a few hundred rounds and functions perfectly unsuppressed. Ejection is a little past 4 o’clock unsuppressed.
For the following shooting evaulations, I used a Harris S-BRM 6-9″ bipod and a Tab Gear rear bag and shot off a bench. I thought about using a tripod and high-power scope but decided against it since I was interested in the accuracy potential I could achieve with the setup as is. So please consider that the groups would likely be smaller if different equipment was used. NRA High Power SR-21 targets were set up at 100 yards. I shot 2 5-shot groups with 3 different types of ammo: our Hornady 75gr BTHP handloads, Atlanta Arms & Ammo 77gr, and PMC X-Tac NATO 62gr. I selected the better group from each type of ammo and used OnTarget to measure them. Here are the results:
The best 5-shot group measured 1.14 MOA with our Hornady handloads and even the worst shooting ammo, PMC 62gr NATO, shot around 1.6 MOA which I think is solid for bulk ammo. Overall I feel accuracy is good considering the use of a 5x scope. I also liked how it shot each type of ammo pretty well too.
To measure the velocity of the shots from our 11.5″ DD and 12.5″ Centurion Arms barrels I used a Magnetospeed V3 chronograph with a rail attachment. Both uppers have a 10″ DD Lite rail so the chronographing setup was pretty much identical. However, the Magnetospeed wasn’t functioning perfectly as I had to play with sensitivity quite a bit and got two velocities per shot, one realistic and one highly unrealistic (such as a velocity in the 4000 fps+ range). I chose the realistic velocity from each shot though and the results are as follows:
The chronograph results do suggest that the Centurion Arms barrel is faster than expected, at least compared with the Daniel Defense. You might expect a velocity increase of 25-50 fps for an extra inch of barrel but the Centurion Arms barrel is 69 fps faster with the Hornady handloads, 54 fps faster with the 77gr Atlanta Arms, and 102 fps faster with the PMC. The Atlanta Arms difference is more in line with the expected increase but the Hornady is a decent amount faster and the PMC is much faster than expected. Nothing too drastic but it’s nice to have that extra velocity bonus, even if it isn’t much, on such a short barrel. It would’ve been best to compare the Centurion Arms with a different 12.5″ barrel but that wasn’t a possibility for me so comparing it to the DD was the best I could do. With the 77gr Atlanta Arms ammo, this extra velocity increase amounts to about an extra 30 yards of fragmentation range (> 2100 fps with 77gr BTHPs). With the 11.5″ Daniel Defense you might expect 140 yards whereas with the 12.5″ Centurion Arms you might expect 170 yards before the bullet fails to fragment.
Overall I thought the carbine shot good groups considering it doesn’t have a match barrel and had decent velocity despite its length but I wanted to see how it would do at longer distances. We took the carbine out to a local range on a nice 57 degree F day with a 5-8 mph wind and set up a MGM Targets 2/3 IPSC steel target at 630 yards. The width of the target is a little less than 12″. I used our 75gr Hornady handloads and shot from a “bench” which was basically a reinforced table and used a bipod and rear bag. I plugged the numbers into the ballistics app Shooter and dialed the elevation and windage corrections on our Burris XTR II 1-5x scope. I dialed instead of held with the reticle since I thought dialing would be more precise as I could just hold dead center in the reticle. This was really easy to do with the exposed target turrets, which are very tactile and in .1 mil/click increments. I was on the target within a couple shots and after a couple more confirmation shots we recorded this string, where you can actually see the bullets hit the target:
The wind was blowing left to right and it looked like I had a bit too much windage on the scope in the first string. We set up the camera on a different spotting scope to get a better view of the trace but it was too shaky on the tripod. The wind condition looked the same so I took .2 mils off and shot this string:
After getting some hits at 630 yards we decided to step it back to 843 yards on a full-size torso steel target. I could only dial 9.3 mils of elevation on the scope, which only went out to 760 yards or so. I decided the next best sight picture would’ve been to hold on the target with the 600y holdover in the Burris so I plugged in different numbers of elevation offset into Strelok+ until I found that putting 7.4 mils of elevation on the scope allowed me to do what just that. I put the elevation and windage corrections on the gun and fired this string:
Hits looked decent at 843 yards but the bullets looked out of gas. The ballistics app Shooter estimated that that bullet was going about 1050 fps or so at impact and only had about 180 foot-pounds of energy. Not much for any serious use but enough for my intended use of long range target shooting on steel.
I’m happy with our 12.5″ build and glad it could still get it done at long range. I wasn’t sure how the barrel length would do at distance but it worked out fine. The Centurion Arms barrel is accurate and fast enough for my use. The carbine feels handy and is very shootable, very smooth. I wouldn’t change a thing on the carbine. The Burris XTR II 1-5x scope works great and I think it’s still the best 1-nx scope for the money. The target turrets are good for dialing at distance on more precise targets yet they don’t really move accidentally. The illumination is bright and the glass is good, although the clarity around the edges isn’t that great. Overall though, I think the carbine/optic combo worked well. I have no complaints. I know any shooting mistakes were all on me and I think that’s about all you can ask for with any gun.