Does high(er) velocity mean I loaded too hot?

[coloradokevin]

I've been handloading for a while, but I've always done so without the aid of a chronograph (never owned one, but always worked my loads up carefully). I recently had the opportunity to put some of my 8mm Mauser loads over a chronograph while shooting at a local range (thanks to another THR member who had such a tool).

Anyway, I was using published Sierra data, working up to loads that weren't quite to the MAX published data. The loads in question were shot from a Yugoslavian M48 Mauser with a 24" barrel, using 175gr Spitzer Softpoint bullets over H-380 powder. My velocities were very uniform (at least in my opinion), but they certainly exceeded the speeds that I expected to see based on the published data. This raised an eyebrow for me, since I believe there is certainly some correlation between velocity and pressure.



Here's the data that gave me pause:

The Sierra load data states that the starting range for this powder is 44.6 grains, and max loads are 53.4 grains. At max loads the published velocity is 2600 fps.

My chronographed loads were as follows:

52.3 grains of H-380: 2,687 fps average (18fps maximum spread in velocity)

52.5 grains of H-380: 2,712 fps average (13fps max velocity spread, with 3 rounds falling within 2fps of each other)

52.7 grains of H-380 (my Max load): 2,742 fps average (46 fps max velocity spread)


So, my question really comes down to asking why my maximum loads were exceeding published velocities by 142 fps, especially considering that I was loading my ammo at least 0.7-1.2 grains below the published maximum charge weight?

Could this be caused by my choice in brass or primers? Seating depth? Something else entirely? If I'm not getting any pressure signs is it safe to use these loads despite the higher velocity?

NOTE: My seating depth was set to 3.150", though the published data seats to 3.100". In my rifle that seating depth does not put the bullet into the lands. Also, I didn't notice any high pressure signs (at least that I could recognize).

 

[MEHavey]

M48 = 7.92 x 57 mm IS (8 mm Mauser)

For MAX loading, QL indicates .323/175gr SierraSP loaded over 53.4gr H380 at a (suggested) COAL=3.228" produces 2,712fps for 48,327psi out of a 24" barrel.

52.5gr H380 w/ a shorter 3.150" COAL would produce a similar 48,436psi and 2,701fps.
(HOWEVER) 52.5gr VARGET w/ that same shorter 3.150" COAL would produce 56,201psi and 2,800fps

The cartridge's Max Pressure is 56,500psi.

With the usual (required) your-mileage-may-vary lawyer caveats for any reloading data:
Using H380, your results would appear consistent with predictions, and within acceptable pressures.
But your VARGET loads are at MAX pressure (My Hornady manuals list 49.1gr VARGET as max for 170gr bullets, while Hodgdon lists 50.5 for 175's. Was "Varget" a misprint?)

 

[KBintheSLC]

If you are not seeing signs of excess pressure, you may still be within cartridge specs. Just remember, the loads with the highest velocity are not always the ones with the highest peak pressure levels.

+1 for the above post as well. The OAL would play a major role in the equation.

 

[R.W.Dale]

Another factor in the mix is how long is your barrel compared to the barrel used to test the loads in your manual

 

[coloradokevin]

Was "Varget" a misprint?

Yeah, I meant to type H-380, which is what I loaded into these cartridges (for some reason I was in a Varget mindset last night). Sorry for the confusion, I went back and corrected the original post (which stated both powders). According to my data I'd have been way over max if I was using Varget.

Anyway, for the sake of clarification, I used Sierra data for the 175gr Softpoint Spitzer bullet in 8 x 57 mm (JS) Mauser, with H-380 powder. The data ranged from 44.6-53.4 grains of powder, and MY loads ranged from 44.6-52.7 grains. The data I'm using specifies a 3.100" COAL, and my loads were set at 3.150" COAL.

Thanks for catching my error, and sorry for muddying the waters with my typo

 

[MEHavey]

That being the case....

"Using H380, your results would appear consistent with predictions, and within acceptable pressures."

Another testament to internal ballistics programs & chronographs teaming up to let you sleep at night.
(Don'cha jus' luv it when a plan works out?)

 

[coloradokevin]

Using H380, your results would appear consistent with predictions, and within acceptable pressures.
...
Another testament to internal ballistics programs & chronographs teaming up to let you sleep at night.

Thanks for the help.

I'm glad that my range data doesn't seem alarming so far. But, what could explain why my chorongraphed velocities would exceed that of maximum published data, particularly when working below max charge weights by 0.7-1.2 grains of powder? I'll admit that your software came up with more real-world numbers than my paper data, but even then I wasn't ever working with the 53.4 grain max load.

Do you think this could be a primer issue? Brass volume? Rifle issue? Atmospheric conditions? Voodoo (which is my favorite explanation)?

I certainly won't complain that I ended up with better-than-predicted velocities, but I like to understand why that may have happened. I only ask because I like to understand as much as I can about the cause-effect relationship when loading, so that I don't ever turn one of my rifles into a shoulder fired hand grenade.

 

[Steve C]

There are all sorts of things that can explain your velocity difference with the "book".

The temperature you where shooting at. Differences in firearms of the same type and brand will often yield significantly different results over the chrono.

I generally get better than book velocities with my hand gun loads, usually as good as listed out of my 4" barrel even though the data comes from 6" barrel pistols. I attribute this mainly to testing in the warm weather we have in AZ.