I checked a few bullets for depth to lands and found that none of them could be loaded in the magazine of my S&W, M&P15PC-1 Talo Exclusive rifle. This is supposed to be a highly accurate rifle. For example, a Hornady, .224 dia, 75 grain BT A-Max in my rifle has a depth to lands of 1.982" giving me a COL of 2.514". The Hornady manual for this load states a COL 2.390". Even this will not likely fit into a magazine. This was the same for the Sierra 69 grain, HPBT and the Berger 70 grain VLD. The VLD was a bit closer though, by about .050". I realize that single feed is the way to go for bullets loaded very close to the lands, especially for bullets such as Berger Bullets, and I have every intension to do so, but, what about those shorter 55 to 60 grain bullets? Won't they be way, way off the lands to be even close to being accurate? With such a large jump, will that erode my barrel? Should I be concerned?

Probably not worth getting excited about. The long chamber throat is theoretically gonna give you less accuracy than a "tight" chamber throat. In actuality the longer throat is a built-in safety measure in that it accomodates a wider range of bullet weights and ogives, plus gives less chamber pressure for the hotter loads. You can't tell how it will shoot until you experiment with loads and try it.

As for concern over possible gas cutting and excessive barrel wear, the US military considers an M16A2 barrel life to be somewhere between 20,000 and 30,000 rounds, even when fired 50% of the time in 3-round bursts. Is that enough barrel life for your semi-auto used in slow fire???

Sir, have you shot the rifle yet? If not, I'd do so before getting worried. The AR is surprisingly tolerant. My old Holliger competition upper was throated long for 80-grain loads seated longer than magazine length, but also did just fine with Hornady 75-grain HPBTs and Sierra Match King 77s loaded to magazine length. I shot the 75s and 77s at 200 and 300 yards and the 80s at 600.

One thing that will definitely affect your accuracy with the heavy bullets is the gun's rifling rate. You don't say what yours is, but for long bullets such as 75s and 80s, you'll want about 1:8. Some 1:9 barrels will stabilize up to 77-grain bullets, but not all of them will.

And regarding barrel life, commercial match barrels have far shorter useful lives than chrome-lined GI barrels. A good AR match barrel such as a Krieger will typically last about 3,500 rounds before the accuracy drops off to a degree unacceptable to a competitive shooter, and I had a Wilson go south in half that number. GI barrels do not have the same accuracy requirement as match rigs, and the chrome lining greatly increases the barrel's useful lifespan.

Hope this helps, and Semper Fi.

Ron H.

Thanks for the info guys. It has a 1 in 8 twist.