Velocity variance in 450B?

[acekc]

I posted in the AR area that I recently picked up a 20" Bushmaster-branded 450B upper. I'm having some ejection issues that may be due to it being overgassed, but I'm also seeing what seems (to me anyway) to be large swings in velocity in my handloads.

Loads:
Starline Brass (new, run through sizer die and expander die before loading)
250gr GTX bullets
2.25" COAL
CCI SRP
LIL GUN (four rounds each with a charge range of 36.4 to 38.2 in 0.2gr increments)
0.478 taper crimp

Charges were thrown using an Autotrickler which has an accuracy of +/- 0.02gr.

For the ten charge weights described above, four shots had extreme spreads (fastest velocity - slowest velocity) as follows: 30, 98, 76, 45, 82, 77, 33, 76, 97, 94. Velocities were measured with a Magnetospeed chronograph.

The next day I measured a smaller range of charge weights (37.5, 37.6, 37.7) this time with five shot groups. This time I used a LabRadar chronograph. Extreme spreads for these were 87, 129 and 30. Note that the 129 in this group was the same charge weight as the 33 in the series above, which implies that the relatively small ES the previous day was just luck.

I'm admittedly not familiar with this caliber but these seem like really large spreads to me. By way of comparison, I did a similar test on a 6.8 SPC upper recently and got extreme spreads of 21, 3 ,31, 23, 38, 10, 21, 29, 34, 26. So the worst 6.8 ES was only 4 fps more than the best 450B ES, and the best 6.8 ES was 10% of the best 450B ES.

Similar results with two chronographs discounts the idea that the chrono is the problem, and both chronos have been very reliable in the past. Weather was basically the same on both days.

If the upper is overgassed, could that possibly cause these velocity swings? I plan on trying different springs and buffers in an attempt to get the ejection problem under control. For this testing I plan on loading a number of rounds with the same charge weight, and I will chrono to see if different buffer components helps. I wanted to start this thread preemptively however in case people had other ideas on a possible cause.

[Al in Mi]

Try a hotter primer, and see if you can get a tighter crimp on those bullets.

[Texas Sheepdawg]

Taper crimp isn’t good enough. Crimp to .474”. Also. Use hotter primer and start your loads at 38.0 grains of Lil Gun. Work up to 40.0 in 0.3 grain increments. Watch for pressure signs. You’re also probably seeing a lot sooting with your loads as the pressures are likely too low to seal the case against the chamber wall. I noticed the sooting started to wane at around 40.0 grains with a really good crimp. Your crimp is a critical part of this equation. You have to have enough resistance to allow pressures to build and the powder to get ignited. Go read my range reports on the 230 grain FMJs I did a few years ago.
These days I use my LeGendre side crimp for all of my loads just to be sure I get good ignition and to reduce bullet pull in my AR15 platform. Sooting is a sign you’re under pressured.

[Texas Sheepdawg]

Study this.
viewtopic.php?f=5&t=8633

[acekc]

Thanks, this is great info. By "hotter primer" do you mean a different brand or a different type i.e. magnum? The only other brand of small rifle primers I have is Federal, but I do have some CCI magnum primers as well.

One more question: I'm starting to read the thread on the 230 grain bullets and I see that you're going from 38gr to 40gr on them as well. Do the extra 20 grains of the FTX not matter in terms of charge weight?

[Texas Sheepdawg]

Thanks, this is great info. By "hotter primer" do you mean a different brand or a different type i.e. magnum? The only other brand of small rifle primers I have is Federal, but I do have some CCI magnum primers as well.

One more question: I'm starting to read the thread on the 230 grain bullets and I see that you're going from 38gr to 40gr on them as well. Do the extra 20 grains of the FTX not matter in terms of charge weight?

Winchester small rifle primers are pretty hot. But yes, you might want to experiment with the magnum primers. I also use CCI number 41s.

[Hoot]

Thanks, this is great info. By "hotter primer" do you mean a different brand or a different type i.e. magnum? The only other brand of small rifle primers I have is Federal, but I do have some CCI magnum primers as well.

One more question: I'm starting to read the thread on the 230 grain bullets and I see that you're going from 38gr to 40gr on them as well. Do the extra 20 grains of the FTX not matter in terms of charge weight?

If you have CCI 450 primers, they will eliminate the primers being the reason for high (by bottleneck standards) velocity SD. There are many contributing factors to that in this caliber. The 800 pound gorilla in the room is consistent, strong, neck tension. A harder target to hit with calibers that headspace on the case mouth.

Hoot

Looks like I doubled with TSD

[Texas Sheepdawg]

Thanks, this is great info. By "hotter primer" do you mean a different brand or a different type i.e. magnum? The only other brand of small rifle primers I have is Federal, but I do have some CCI magnum primers as well.

One more question: I'm starting to read the thread on the 230 grain bullets and I see that you're going from 38gr to 40gr on them as well. Do the extra 20 grains of the FTX not matter in terms of charge weight?

I’ve tested 40 grains of Lil Gun with the various 230 grain full metal jacket bullets & the 250 grain FTX and shockwave & the spear 250 grain deep curl. My A.R. 15 is a 20” barrel 1:24 twist. My Ruger American is a 16” barrel 1:16 twist. My New Englander is a 20” barrel 1:16 twist.
All three of my rifles like 39.0 to 41.5 grains of little gun with 230 and 250 grain bullets as long as the crimp is superior and the taper crimp is at .474”.
But like I always say, my guns are not like your guns. Everyone’s chambers will be different. Always measure your chamber and make sure your case overall length is right and that your seating depth is right. And a LeGendre Side crimp is just icing on the cake. Especially with the slower burning powders.

[Texas Sheepdawg]

Thanks, this is great info. By "hotter primer" do you mean a different brand or a different type i.e. magnum? The only other brand of small rifle primers I have is Federal, but I do have some CCI magnum primers as well.

One more question: I'm starting to read the thread on the 230 grain bullets and I see that you're going from 38gr to 40gr on them as well. Do the extra 20 grains of the FTX not matter in terms of charge weight?

If you have CCI 450 primers, they will eliminate the primers being the reason for high (by bottleneck standards) velocity SD. There are many contributing factors to that in this caliber. The 800 pound gorilla in the room is consistent, strong, neck tension. A harder target to hit with calibers that headspace on the case mouth.

Hoot

Looks like I doubled with TSD

Yo! Sheepdawg is in Da House!

[acekc]

Well thanks to everyone for the great advice. I made it back out to the range today and shot some new loads:

Starline Brass
250gr FTX
0.474 Taper Crimp
CCI Small Rifle Magnum Primers
2.25" COAL
3 each of 38.2g-40.0gr LIL GUN in 0.2gr increments

With the changes to crimp strength and the switch to magnum primers, extreme spreads came down a lot. For the ten charge weights ES was 33, 31, 46, 23, 64, 9, 11, 29, 30, 16. Needless to say I'm much happier with these numbers.

As TSD noted I did see sporadic sooting with some of the middle loads. It seemed to go away in the 2-3 highest charge weights but in some cases only happened on one of the three shots at a given charge weight.

Unfortunately I'm still having ejection issues, but I'll talk about that in the other thread I started in the AR-15 area.