Is it unusual?

[Cowboy_mo]

Is it unusual to get higher muzzle velocities than the loading manual calls for?

Okay, I loaded some 30-06 recently using IMR 4064 and 150 gr bullets. I loaded some @ 50 grains and some @ 51.7 grains. According to the Hornady recipe the MV for 51.7 grains should have been 2900 fps and the MV for 50 grains should have been around 2800 fps.

My loads tested (avg over 5 shots) 3002 fps for the 50 grain and 3006 fps for the 51.7 grains. Both loads are under the max load the manual calls for and I wasn't trying (nor do I really want to) load them "hot".

None of the rounds showed any excess pressure signs and both loads grouped under 1 inch, so I'm very happy with the performance, but was surprised by the results.

 

[Shoney]

What is the distance from muzzle to chronograph?

 

[Brian Pfleuger]

Those velocities seem rather high but it's dependent on the length of your barrel versus theirs. It's also dependent on the exact bullet. Two 150gr bullets can be quite different in length, therefore effecting pressure if seated to the same length. Distance from the lands will have an effect also.

Also, you say your loads shows no pressure signs but a 1.7gr increase of powder that shows essentially ZERO increase in velocity is a pressure sign in itself.

 

[sserdlihc]

My loads tested (avg over 5 shots) 3002 fps for the 50 grain and 3006 fps for the 51.7 grains. Both loads are under the max load the manual calls for and I wasn't trying (nor do I really want to) load them "hot".

1.7 grains of powder only yielded a difference of 4 fps. That is almost nonexistent. Something doesn't seem right.

If I may ask, Why the jump from 50 grains to 51.7? I have heard reloaders increasing in .5 grain increments, even know one fella on TFL that increases at .3 grain increments. I increase at .5 grains myself. Seems a little unorthodox.

 

[Cowboy_mo]

Tried to cover everyone's questions.

1. Muzzle to chrono distance was about 10-15 feet.
2. I thought the minor difference between the 2 loads was also strange.
3. The difference in the loads because the lower was supposed to produce 2800 fps and the higher was supposed to produce 2900 fps (what most of my factory ammo calls for). It was a bit of an unorthodox comparison test, I admit.
4. The bullets were the same Hornady 150 gr. out of the same factory box.

 

[Brian Pfleuger]

The bullets were the same Hornady 150 gr. out of the same factory box.

I meant the difference between the 150gr bullets used in the published data and those used in your gun.

QuickLoad lists 150gr .308 bullets that range in length from .831" to 1.350". Most are in the vicinity of 1.1" but if the data in the manual was produced with a bullet of 1.05 and your bullet is 1.25, you could be looking at a 5,000 psi difference if they're seated to the same OAL. That difference alone could account for 75 fps.

Also, what was the difference in barrel length between the gun used in the published data and your gun? It wouldn't be at all unusual to see 50+ fps per inch of barrel length.

 

[Cowboy_mo]

Published Data

The published data was from the 7th edition Hornady Handbook of Cartridge Reloading as follows:

Rifle: Winchester Model 70 23 3/4" barrel, 1 in 10" twist
Case: Hornady /Frontier
Primer: Federal 210
Bullets: 150-155 grain Hornady bullets

My rifle and loads comparison
Rifle: T/C Venture 24" barrel, 1 in 10" twist
Case: WWC 53 (military cases)
Primer: CCI 200
Bullets: 150 grain Hornady BTSP

I am relatively new at reloading so I checked all the things that I see talked about most on this forum and in my reloading manuals to try and explain why I got higher than expected (from published data) velocities and why the two loads velocities were almost identical. I guess a different primer and/or case could account for it but it just seemed strange to me.

 

[Brian Pfleuger]

I believe that military cases generally have less internal volume than civilian cases.... same outside dimensions, thicker brass equals less volume. Less volume equals more pressure at the same load levels. More pressure equals more velocity.

 

[mehavey]

It is a bit high. QL would predict ~2,870fps for those criteria assuming 51.7gr of IMR4064 in a 68.2cc case, 3.23" OAL (at cannelure), and a 24" barrel. It would push ~51.6ksi load

Coupla things in order of increasing probability:
- Bad/fast batch/lot of IMR4064 (have a burn rate more than >15% over nominal -- very unlikely)
- Military case having less capacity than federal commercial (normally not a "big" effect, maybe 20-25fps)
- Tight chamber (ditto above, another 20fps)
- Tight barrel (can have greater effect)

Compound a few of these things and you can see how you might experience what you're seeing. Assuming the chrono is giving you nominally true readings, ballistics would predict you are approaching pressures slightly over max (~62ksi)

My counsel would be to use the chronograph to bring the load down to the mid/high 2800's. Based on the above an QL, that would likely be ~48.5-49.0gr at ~52,000/53,000 psi.

 

[medalguy]

Here's another thought. I have two identical chrono's because I have two homes and I want one at each. Well recently I managed to get both of them together, and just for chuckles I put one right behind the other and tried shooting through both chronos to compare results. Surprise-- the newer chrono reads 6% faster than the older one. The results were consistent for both pistol and rifle trials, so this may have some impact on your own observations.

 

[DaleA]

Just tossing factors into the discussion, temperature can play a part too. If the ammunition is hot (sitting in a vehicle in the sun might do it) you can get higher velocities.

That still doesn't explain why the two different loads gave you almost the same velocity readings.

Medalguy-that was a neat test of the chronographs.