New rifle eject issues

Talk about the AR15 style rifles chambered in 450 Bushmaster.

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Re: New rifle eject issues

Postby plant_one » Tue Jan 02, 2018 9:42 am

Hoot wrote:
I am surprised that you clocked them so fast. I forget, were you using a stab crimp or just the taper crimp. Not that it matters because if you're getting 2k fps, you're not getting that with only 22kpsi. Can't happen with a 300 grainer in this caliber. As a matter of fact, I went off the reservation and tweaked the QL equates to get a 300gr bullet up to 2k fps in this caliber. Not hard with slower powders and longer barrels, but with Lil Gun quickly pushing the bullet at that speed, regardless of how you got it to do that, you will be up into high pressure.

Hoot



i dont have my notes in front of me, but a max charge of lil gun was just over 2000 fps with the 300gr XTP mag in my 20" bushmaster barrel according to magnetospeed

didnt shoot for Poo, but it was cruising along and man did it have some thump to it on the recoil side :mrgreen:

[edit] taper crimped to 0.474
Last edited by plant_one on Tue Jan 02, 2018 1:44 pm, edited 1 time in total.
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Re: New rifle eject issues

Postby Rklenke » Tue Jan 02, 2018 10:59 am

I got around to measuring the cases, and measured unfired Starline and Hornady for control. The numbers look inconclusive to me, but maybe you will see something I don't. Some of the numbers are average as the cases weren't perfectly round. I was using the wide part of the caliper so the measurement at the case head looks really big. If i used the narrow part and placed it as close to the extractor groove as possible, the Black measured at an average of .499" and the reloads measured an average of .498".

Code: Select all
                    New Black   Fired Black    New Starline   Good extract          No extract
Case Mouth            0.478           0.476       0.478              0.479           0.479
2                     0.478           0.479       0.478              0.481           0.481
3                     0.480           0.482       0.480              0.484           0.483
4                     0.483           0.487       0.483              0.488           0.484
5                     0.486           0.488       0.487              0.490           0.489
6                     0.489           0.492       0.490              0.492           0.492
7                     0.491           0.494       0.493              0.495           0.495
8                     0.497           0.498       0.495              0.499           0.499
Case Head             0.498           0.501       0.497              0.502           0.502


I've got some 230 grain FMJ coming from RMR, I'll have to slug the bore with one of those when they show up. In the meantime I'll be shopping for 1680. I really like the AA9 performance out of my 10mm, so I've got to believe the AA offering will be good in the 450BM as well.
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Re: New rifle eject issues

Postby Bmt85 » Tue Jan 02, 2018 2:26 pm

You really need a mic to get real numbers, I personally use a blade mic. Your looking for numbers down to .0001, I have a set of calipers accurate within .0005, and don't use them for checking case head growth.
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Re: New rifle eject issues

Postby Rklenke » Tue Jan 02, 2018 2:33 pm

That table looks terrible on my phone. It was all lined up on my computer. Hopefully it's readable.
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Re: New rifle eject issues

Postby Hoot » Tue Jan 02, 2018 2:37 pm

Where I came up with 22k psi initially using Lil Gun, I was using a COL of 2.26. When I changed it to 2.08, that jacked the pressure = velocity up using QuickLoad software. I still could not get it up near 2k fps without increasing the start pressure up to that of a solid copper (Aka Barnes) bullet. That starts with 3 times the start pressure as a pistol cup and core bullet in the QuickLoad program. Don't confuse start pressure with max chamber pressure. Start pressure is a constant in the equation that accounts for how hard it is to squeeze the bullet down to the dimensions of the bore. That's what leads me to suspect a dimensional issue with the bore or chamber. The rule of thumb with barrel manufacturers AFAIK is to use a bore of .4515. That accommodates both .451 and .452 bullets. Something just occurred to me. Have you measured the actual diameter of a couple of your 300gr bullets? Despite manfacturers going to great lengths to control the quality, I have seen one or two lots of bullets over the years, that were not as specified using a calibrated, precision micrometer.

I was goofing around last night about punishing the barrel. I did sell the one with the tight chamber after I corrected it, but that was to fund a different twist rate barrel purchase. ;)

1680 is a good powder for less than peak performance in 300 grainers. It works but is a little on the slow side for peak performance, especially from the carbine length (16in) barrel. That's why you wind up flinging a little unburned powder downrange, even with a 20in rifle length barrel. H110/W296/300MP work pretty well with the 300gr bullets. I liked 300MP the best when I was testing 300gr bullets.

+1 to what Bmt85 said.

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Re: New rifle eject issues

Postby plant_one » Wed Jan 03, 2018 12:00 pm

ya 1680 wont burn completely until you start to compress it - or at least in other calibers. i havent tried to mess with it yet here in the 450. busy summer last year, i got almost no load developement/testing done :(
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Re: New rifle eject issues

Postby Rklenke » Sun Jan 07, 2018 9:31 pm

I haven't gotten out to shoot much in the cold, but I was thinking as I read Hoots cheap carrier weight post: if I use a heavier buffer, will that delay unlocking until the pressure is lower and allow for a better extract? Right now I have a "heavier than carbine" buffer, not sure which H designation it is. I also have a 9mm buffer that I could either try or take weights from to tune my current buffer. Thoughts?
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Re: New rifle eject issues

Postby Rklenke » Sun Jan 14, 2018 3:50 pm

There was no difference using a heavier buffer. Every Hornady that I shot extracted fine, every reload required hand extraction. I'm just going to have to accept that somehow the reloads are too hot even though they are per the Hornady manual.
IMG_20180114_163434407_BURST000_COVER_TOP~2-402x448.jpg
IMG_20180114_163434407_BURST000_COVER_TOP~2-402x448.jpg (42.26 KiB) Viewed 75 times


All of the Hornady Black had pretty heavy sooting while the reloads were almost perfectly clean, which I guess still points to a higher pressure.
IMG_20180114_162229940_BURST000_COVER_TOP~2-402x403.jpg
IMG_20180114_162229940_BURST000_COVER_TOP~2-402x403.jpg (38.98 KiB) Viewed 75 times


I'll just load some lighter ones, and one of these days I'll find 1680 locally to try out.
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Re: New rifle eject issues

Postby Bmt85 » Sun Jan 14, 2018 7:26 pm

So I'm assuming that in the second pic, the Hornady and Starline cases are swapped, so the Starline (which is cleaner) is on the left, and the Hornady (which is dirtier) is on the right. Just to make sure we are on the same base.

Now, the primer on the Starline looks a little flattened, which indeed would point to over pressure.

As to the sooting, I usually see it on factory ammo, or on the low end of reloads. With less sooting, you have a good seal, which is what you want, as long as your not overpressure that is. One thing I have noticed with new Starline brass, is it seals faster than Hornady, i.e. on a lesser powder charge.

I actually came across a similar issue to yours in my Tromix build. I was using new Starline brass and doing a load work up on 275 TSX and Lilgun. I was running into an issue with the brass growing too much and preventing cycling. Case head measurements were a little high, but nothing crazy. I had to mortar the rifle to get the case out. When I measured the case, it was right at max allowable, after sizing it was over. From what I can tell, Tromix barrels have a slightly tighter chamber and bore, because they also produce faster velocities compared to other barrels.

So, I think what's happening is you have a tighter chamber and bore, and Lilgun is then running over pressure, causing your new brass to grow too much and is seizing the bolt. Just a guess.
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Re: New rifle eject issues

Postby Rklenke » Sun Jan 14, 2018 7:49 pm

I bet you're right on, and that is what hoot suggested as well. Once my FMJs come in I'll be able to get a measurement on the bore to see if it's closer to .451 or .452. For now I've already reduced the charge and will be headed out tomorrow to see if it cycles.

On a side note, I sized all my brass today, and the Hornady brass ran through the die noticeably easier than the Starline.

I wish this had started happening when I first worked up the load, but I suspect a 50 degree temperature swing affects the chamber tightness as well.
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