01-23-2014, 10:42 PM
LRRPF52 Wrote:I just went and double-checked my known AA SAAMI chamber, and a box of 123gr Hornady SST factory ammunition, as well as my 123gr A-MAX hand loads that are at 2.272"-2.275" COL.
With the 123gr SST's, raw COL comes out to 2.232" to 2.240". How much of that is polymer meplat vs. ogive and seating depth variation, who knows.
I blackened my ogive just above the cannelure, dropped a loaded factory 123gr SST in my chamber with the upper receiver oriented vertically, and removed the case. I repeated this several times. I was able to barely detect the slightest hint of engagement with the cone in the Grendel chamber, but we're talking just the ever-so-slightest kiss.
I then grabbed my 123gr A-MAX hand loads that are at 2.272" COL, and there was little, if any forcing cone kiss. We are nowhere near the actual lands, but are at the hand-off point for the forcing cone, where the angle begins. From a physics standpoint, this very slight angle accomplishes a few things.
Regarding pressure, you have a nice transition from the release out of the neck tension, right into a uniform bearing surface, rather than a jump.
For a gas gun with magazine OAL and reliability considerations, the forcing cone makes a lot of sense. One of the biggest complaints you will see with factory gas guns that people are using to chase accuracy, is that there is too much jump to the lands from even the maximum magazine allowable OAL. No such problem with the SAAMI Grendel chamber. Combined with the .300" neck in the chamber, which allows for reliable chambering in even adverse conditions with a gasser, the forcing cone handles a wide range of projectiles well.
The forcing cone also centers the projectile, regardless of how it is oriented in the cartridge & chamber. Most accuracy seekers approach this issue with a tighter neck, but that isn't the best method in a self-loader, as I have learned with my .260 Rem AR10. Only way I can load and shoot factory brass is if I size all the brass before loading. The Grendel is much more forgiving in this regard.
Is that a brand new unfired AA barrel? If not is it possible it was more than a kiss on the first date. After being fired "X' numbers of times your virgin barrel might not be as tight after the first load was let loose. Use the barrel and see how it is. If it wont chamber with factory ammo, send it back for investigation. And no it wont just slip right in the first time, it will need a little finesse or thrust.
As J.A. Smith said, sit back and look at the info. I have looked at the drawing for hours and discussed with many people in depth. Read the numbers of the throat. Right under the case end measurement (.2644) is shows the next measurement of the free-bore ( .2617). That is a smaller number than .264 so it will touch, kiss, a .264 diameter bullet.