Initial loading issues

[interceptor]

Originally Posted by jinx44

So when I stick a bullet in the barrel, it will cycle.

So hows you accuracy at that point?


IMO, you're going in the wrong direction with your loads, down instead of up. You describe hearing a crack when shooting... Next time out let a partner do the shooting and you stand 50-60 feet away, see what you hear. I think you'll be suprised at how quiet it is.

My load is 11.8g AA1680 under 220 SMK's, and from 100 ft away (with YHM can) it's like an air gun. AND IT CYCLES THE BOLT, FULLY, CHAMBERING ANOTHER ROUND. I do run a carbine spring in a rifle length tube with a 88gram buffer.

[pomofo]

At 6.0 grains, Quickload is showing 981fps, kind of curious that you're still getting a crack. The 11.8 grain load with 1680 that was mentioned earlier is supposed to be getting 1251fps (16" barrel), loaded to mag length. The problem with 300 Whisper is that you're using relatively small amounts of fast powder to push heavy bullets. Chamber pressure may be high, but it diminishes rapidly.

Quickload also provides estimates of muzzle pressure, which indicates how much pressure is lost in travelling through the barrel. That 1680 load is supposed to be ~2700psi at the muzzle, as opposed to 1900 for your 8.4gn of AA9, and 1500 for your 6.0gn. By comparison, 5.45x39 still has about 11000psi at the muzzle. Muzzle pressure curves are parabolic, so they increase and decrease at some exponential rate. My guess is that if you're losing 40000psi over 16", much of that came before you hit the gas port at 7".

[jinx44]

Originally Posted by interceptor

So hows you accuracy at that point?

Well, I am pretty sure if I would have left that bullet in the barrel, and shot another round, it would be accurate enough to stack bullets on top of one another. LOL

The crack I am hearing is out away from the gun, as in it sounds like its a hundred yards away. The sound of the rifle isn't loud at all.

I am trying to dig up a chrono, so I will see exactly where I am at. I really appreciate the help you all are giving me, and being patient with a newbie.

[Flewis]

For what its worth, I'm using 11.6 grains of AA-1680 behind a 220 SMK and am getting 1080 fps. Cycles the bolt every time and locks back on an empty mag. I have a 16" barrel with the gas port at the carbine position. I never had any luck with H110... never would cycle either of my rifles. Oh, and my gas port is 0.125.

[jinx44]

I had a chance to play with some 1680 yesterday. Cycling problem has been fixed with this powder. I started with 11.0 gr and still heard a crack. 45* air, and I think we are roughly 450' elevation.

Though according to this calculator, elevation is not important: http://www.sengpielaudio.com/calculator-speedsound.htm

By putting 45* in there, SOS is 1101 ft/sec. Fired brass on these winchester casings are showing the neck is not too tight. I am still loading at 2.110", which is .002" off the lands. I have thought about seating deeper incase that is causing a pressure rise, but I feel they are short at that length anyway (which may be causing higher pressure).

My next step is to get a chrono to verify if my ears are deceiving me. Now I just have to decide which one. I have been reading good things about the CED Millenium. Opinions?

[Flewis]

Are you sure you aren't hearing the report of the rifle bouncing back to you from a hillside or berm and mistaking that for super-sonic crack? With 11 grains of 1680 I'd be really surprised if you are going super-sonic.

[redtazdog]

Originally Posted by Flewis

Are you sure you aren't hearing the report of the rifle bouncing back to you from a hillside or berm and mistaking that for super-sonic crack? With 11 grains of 1680 I'd be really surprised if you are going super-sonic.

I was thinking the same thing because I use 11.4 grns of 1680 and get
1040 fps through the pact chroni.

[jinx44]

Originally Posted by Flewis

Are you sure you aren't hearing the report of the rifle bouncing back to you from a hillside or berm and mistaking that for super-sonic crack? With 11 grains of 1680 I'd be really surprised if you are going super-sonic.

I have been wondering the same. Tonight I am thinking of going to 12.2 grs, which should definetly be supersonic and compare the sounds.

[Pitt300]

If you're hearing a "reflected crack" from the targeted background, that's supersonic crack.

Normally you will not hear the "boom" of the gun being fired as a "high" reflected sound.

Have someone stand away from you when you shoot & see if they hear a "crack" down range.

[jinx44]

Since progress has been put on hold until I can get my hands on a chronograph, I thought I would make a change that I have been wanting to do. When I picked up the parts for the build, I was able to get a billet Sun Devil lower for the same price as a RRA. I liked the tension screw in the Sun Devil that can tighten against the rear pin to tighten the receivers together. The upper was just a standard J&T flatop with forward assist. I went back last night and picked up a Sun Devil billet slick side upper and swapped them out. Even before adjusting the tension screw, this is BY FAR the tightest AR I have ever built. They make for a very solid gun.

Here are some pictures:










Internal of the SD lower. Note the tension screw with the green face in the back:




Look at how beefy the upper is:




Picture of the tension screw head with the grip removed:





I ordered another set of Sun Devil receivers, blue anodized, for a 6.5 Grendel build.