Velocity went up on 5.56 2 yr. old reloads - Why???

[MMH]

I have some 5.56 reloads that are shooting faster than they did 2 yrs. ago. Here are the load details:
Brass: LC15
Bullet Hornady 75 gr. BTHP
Powder: 23.5 gr. of 8208 XBR

Average velocity (22" barrel) is 2894 fps & ES (5 shots) is 9 fps
This is about 100 fps faster than the velocity that I had 2 years ago.

I did wet tumble the brass w/ SS pins. I have been reading that this may cause bullets to stick - could this be the cause? When I recently checked the velocity it was 70F outside. Don't remember what it was 2 years ago but guessing that it was colder - probably 40F or so. Is 8208 temperature sensitive?

Anyway, happy w/ extreme spread just confused why the velocity went up and with that velocity not getting close to transonic at 1000 yards.

 

[TIMO]

Bullet weld, that I have read about here on AS.
Tim

 

[jelenko]

What temp did you shoot at this year? No question velocity is going to go up with temp.

Is it correct that you are shooting the loads out of the same rifle with the same barrel? Are you seating the bullets to the same depth?

FYI. 2894 MV on an H 75HPBT is not going to be super sonic at 1000 yards unless you're up around 5000 to 6000 ft above sea level.

 

[rardoin]

State of carbon/copper deposition in your barrel could be a cause as well as environmental conditions.

 

[FeMan]

Different ambient temps as stated above are a likely cause. If its a newer barrel, it could have also sped up.

 

[MMH]

What temp did you shoot at this year? No question velocity is going to go up with temp.

Is it correct that you are shooting the loads out of the same rifle with the same barrel? Are you seating the bullets to the same depth?

FYI. 2894 MV on an H 75HPBT is not going to be super sonic at 1000 yards unless you're up around 5000 to 6000 ft above sea level.

JBM at 1342 feet elevation & using a G1 of 0.395 indicates that bullet will be 1216 fps (mach 1.086).

 

[WVRJ]

Many variables can cause velocity to change.Temperature,lighting for chronograph,different chronograph(they can vary,especially the little short ones),bullets sticking in the brass,barrel fouling are a few things that will change it.

 

[sawcarver]

JBM at 1342 feet elevation & using a G1 of 0.395 indicates that bullet will be 1216 fps (mach 1.086).

1340-900fps is transonic so you're there

 

[jelenko]

JBM at 1342 feet elevation & using a G1 of 0.395 indicates that bullet will be 1216 fps (mach 1.086).

Hmmm. Using same JBM, at 4000 ft, using either .395 G1 or just accepting the default after choosing Hornady 75gr HPBT from the drop down list, it's 1097 fps or mach .983.

Manufacturer:	Hornady	Description:	Boat Tail Hollow Point Match (Litz)
Caliber:	0.224 in	Weight:	75.0 gr
Ballistic Coefficient:	0.183 G7 (ICAO)
Muzzle Velocity:	2894.0 ft/s	Distance to Chronograph:	10.0 ft
Sight Height:	1.50 in	Sight Offset:	0.00 in
Zero Height:	0.00 in	Zero Offset:	0.00 in
Windage:	0.000 MOA	Elevation:	0.000 MOA
Line Of Sight Angle:	0.0 deg	Cant Angle:	0.0 deg
Wind Speed:	5.0 mph	Wind Angle:	90.0 deg
Target Speed:	10.0 mph	Target Angle:	90.0 deg
Target Height:	12.0 in
Temperature:	59.0 °F	Pressure:	29.92 in Hg
Humidity:	0 %	Altitude:	4000.0 ft

Range	Drop	Drop	Windage	Windage	Velocity	Mach	Energy	Time	Lead	Lead
(yd)	(in)	(MOA)	(in)	(MOA)	(ft/s)	(none)	(ft•lbs)	(s)	(in)	(MOA)
0	-1.5	***	0.0	***	2901.5	2.599	1401.8	0.000	0.0	***
100	-0.0	-0.0	0.4	0.4	2679.4	2.400	1195.3	0.108	18.9	18.1
200	-3.4	-1.6	1.5	0.7	2466.4	2.209	1012.9	0.224	39.5	18.8
300	-12.4	-4.0	3.6	1.2	2263.2	2.027	852.9	0.351	61.8	19.7
400	-28.3	-6.8	6.7	1.6	2070.3	1.854	713.7	0.490	86.2	20.6
500	-52.3	-10.0	11.0	2.1	1886.8	1.690	592.8	0.642	112.9	21.6
600	-86.2	-13.7	16.6	2.6	1711.4	1.533	487.7	0.809	142.3	22.7
700	-131.9	-18.0	23.7	3.2	1543.4	1.382	396.7	0.993	174.8	23.8
800	-192.3	-23.0	32.7	3.9	1383.1	1.239	318.5	1.199	210.9	25.2
900	-271.0	-28.7	43.8	4.6	1231.9	1.103	252.7	1.428	251.4	26.7
1000	-372.7	-35.6	57.5	5.5	1097.4	0.983	200.5	1.687	296.9	28.4

 

[Dusty Stevens]

One thing always stays the same- velocity changes all the time

 

[riflewoman]

I seriously doubt if the bullets welded in any significant way. If it had, theES would be way higher. The most likely reason is a slight tightening of the neck over time.

 

[piie]

You've got a good cultivar there, it's aging well - stop complaining

 

[RegionRat]

I have had batches I let sit for months and into years just to test their stability. I'm still playing with some that date back to the 80's and 90's.

In some examples, I had poor performance, and in others good.

I am not ready to preach about how to guarantee things will always come out good, but I can also say that there was definite correlation between the bad ones changing the breakaway seating force versus the good ones being stable.

When I had batches that were good in tests when they were fresh that later went bad, their seating force friction tests correlated when I measured them on a K&M arbor press with a force pack. (See more comments below about breaking them loose),

For example, their seating force break friction more than doubled on some bullets and with some pegging the force pack so I don't know how high they were. When freshly loaded and testing good, their seating force stats were good with an average of about 40 - 45 lbs. When "bad" their values would jump from 40 - 45 up to 90 or more. You could really feel them stick and slip.

I don't remember how many decades ago I was at some national level match and the guys were assembling their ammo long, and seating it to final dimensions at the match. Their reasoning was to minimize these effects.

Those rounds I tested for seating force that pegged the gage and "popped loose", came back to good friction levels and then also tested good for velocity and on the targets too.

I've been loading for hunting trips and traveling to matches for many decades now, and I load the ammo long and seat it when I'm ready. I think I have it figured out because I haven't had a "pop" in a long time. However if it is an important hunt or match and the ammo has been stored for months, I still load it long.

Loading and testing ammo in nice weather, and then hunting in -20 is a different and much longer discussion. You always have to account for temp swings and altitude. But, the storage stability of your neck prep is a different story.
YMMV

 

[MMH]

So I seated the bullets 0.010" deeper and now have:
Average velocity is 2862 fps & ES (5 shots) is 30 fps.

Reshot the original version (i.e. bullets not reseated) and got:
Average velocity is 2870 fps & ES (5 shots) is 30 fps.

So, I do not believe that the bullet was stuck/cold welded. Avg. vel. for 15 rounds is 2875 which is 75 fps faster than what it was 2 years ago. Will throw 5 rounds in the freezer to check temp stability.

Maybe the barrel got faster - it was fairly new 2 years ago and now has 400 rounds through it. It is a Bartlein 1/7 4 groove SS barrel.

 

[cw308]

First thing I thought of was bullet weld also.

 

[MMH]

Hmmm. Using same JBM, at 4000 ft, using either .395 G1 or just accepting the default after choosing Hornady 75gr HPBT from the drop down list, it's 1097 fps or mach .983.

Manufacturer:	Hornady	Description:	Boat Tail Hollow Point Match (Litz)
Caliber:	0.224 in	Weight:	75.0 gr
Ballistic Coefficient:	0.183 G7 (ICAO)

You used the same JBM, but used the library to pull in Litz data for that bullet. Some of your parameters were a little off from mine but I reran w/ your parameters to compare the Litz prediction to the G1=0.395. The difference at 1000 yards was significant - Litz values predicted a velocity of 1097 fps, G1=0.395 predicted 1216 fps. Never would have expected such a large variation. The ultimate test will be bullet dope data. I am going to the range tomorrow & will check drop from 300 yards all the way out to 1000 in 100 yard increments.

EDIT: had wrong value for G1 prediction - should have bee 1216 fps.

 

[CharlieNC]

I have experienced cold welds with Hornady bullets, no others. They were so bad I heard a "pop" noise as the broke loose when I seated them deeper.

 

[Pawnee Bill]

Aliens

 

[MMH]

Went back out today. Verified my velocity for the third time. Shooting at 2875 fps, which is about 75 fps faster than when the barrel was new. Also, started using a G7 = .183 instead of the G1 = .395. Shot from 300 to 1000 yards and predicted drop was close to calculations, but not perfect. However, wind was significant, varying from a standstill to 12 mph and going all the way around the clock. Mostly either at 12 or 6 o'clock, but sidewind was significant as it wandered around. Rifle shot good to 900 yards, but at 1000, my vertical spread was over 3'. Thinking that wind combined with bullet going subsonic is making 1000 yds. a challenge. I may only have a 900 yard rifle. I guess that's OK as I'm only a 600 yard shooter...