300 wm speed variation

[boeger42]

Problem: I have a new 300 wm with just over 200 rounds so far. I'm unable to duplicate speeds and SD with any consistency. H1000 works well on new cases but looses it after being fired. Load specs are as follows:

Berger 190 VLDs
Norma Brass - fire formed and full cases there after
Headspace 2.298 = zero headspace to shoulder
OAL 2.765 = .012 off the lands at ojive
Runout > .003
Fed215M primers

I use 3 round groups as I figure if the load can't hold SD at 3 rounds it won't hold it at 5 or more? Group size consistently correlates well to chronographed speeds; however, I'm unable to duplicate the speeds.

I've tried H1000 and now IMR4350. Here is one result:
9/24/10 100 yds:
69.1 gr of IMR4350 yields an average speed of 3018fps, SD of 4, ES of 7fps and group size .317 ctc.

69.5 gr of IMR4350 yields an average speed of 3000fps, SD of 14, ES of 27fps and group size of .7545 ctc.

69.9 gr of IMR4350 yields an average speed of 3018fps, SD of 4, ES of 7fps and group size of .194 ctc.

9/25/10 same bench, environment, and range:
69.9 gr of IMR4350 yields an average speed of 3080fps, SD of 37.1, ES of 78 and group size of .7. I didn't bother to measure. The chrono was suspect as it failed to detect the first 3 rounds and displayed a duplicate speed without indicating dup; however, the group size confirms that there was too much speed variation...

Speculations, Suggestions, Psychiatric Counseling?

 

[Victor N TN]

Re: 300 wm speed variation

Your message indicates possible past chronograph problems. May I ask what brand and model chronograph you are using?

When I want accurate load data I shoot minimum 10 round strings / groups. To shoot groups of lesser quantity is just inserting potential problems into the string. But unless you are shooting 1K competition or something close, a Standard Deviation of anything under 20 is wonderful. (Standard Deviation is the number you want to look at. The Extreme Spread is just something to write down and use to look for mistakes in what did you did to that specific round or rounds.) Both the powders you list have served me well for quiet a while.

If your chronograph is of a better known brand name like Oehler, Pact or CED shooting a longer string should even out the numbers. Re-read your chronograph's instructions about how far down range to set up the sky screens. Setting them closer than 10 feet can cause a lot of false readings generated by the preceding gasses of the muzzle blast. Personally I set them out at 10 feet for pistol loads and out as far as 18 feet with magnum loads like the 300WinMag. (That's as far as the factory made wires will let me move it.)

I hope this helps solve your delima.

 

[boeger42]

Re: 300 wm speed variation

Thanks for that Victor...

I've been using the cheepy prochronograph that's already set up at the range. It sets about 15 feet in front of the bench. I'm going to try to proof the 69.1 load of IMR4350 and a slower load that worked well earlier. If nothing pans out, I've got some new brass that I'm going to start over with. While fire forming I got several SD's below 10 using H1000 but none of them were over 2900fps. Worst case I can start resizing the brass back to unfired dimensions. That's bumping the neck back .018 from where the chamber puts it.

I've got a chrony but it's time to buy a real chrono. It's on my list. Next year I would like to have a 1k trajectory documented so I could contimplate the taking of an animal at 800+. We're off to WY for a deer hunt in a couple of weeks so I won't get out till we get back. I'll let you know what turns up...thanks!

 

[Victor N TN]

Re: 300 wm speed variation

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: boeger42</div><div class="ubbcode-body">Thanks for that Victor...

...Worst case I can start resizing the brass back to unfired dimensions. That's bumping the neck back .018 from where the chamber puts it.
</div></div>

Please tell me that .018" (eighteen thousandths) is a typographical error. Would you mean .0018" (one thousandth and eight tenths) ? If you are pushing the shoulder back .018" (eighteen thousandths) something is wrong somewhere.

 

[boeger42]

Re: 300 wm speed variation

.018 is the difference between the head space dimension of the unfired brass and the head space dimension of a fire formed case.

Norma unfired headspace dimension(shoulder) = 2.280
Fire formed headspace dimension(shoulder) = 2.298

I'm currently not pushing the shoulder back at all. I've been running all my reloads at 2.298...

I don't know why manufacturers give so much head space on the unfired brass but I hear they all do it for the belted mags?

 

[Hummer]

Re: 300 wm speed variation

Such a big shoulder movement on fire formed cases is completely normal in 300 Win Mag. That is why the L.E. Wilson Magnum Case Gage is adjustable.
I set my 300 Win Mag to just bump the shoulder back .001 to .002". No problems and longer case life.

 

[Fuzzball]

Re: 300 wm speed variation

" the group size confirms that there was too much speed variation..."

You mentioned shooting at 100 yds on that bench. How would your group size confirm a speed variation at that distance?

 

[boeger42]

Re: 300 wm speed variation

Hummer, what do you use to measure your headspace?

 

[boeger42]

Re: 300 wm speed variation

Fuzzball,
I sent you a long response but don't know what happened to it?

Anyway, there is a statistical correlation between group size and velocity on this rifle. I haven't run the numbers but when the SD and ES are low the groups are tight. It shoots a near perfect group one day then all over the place the next day. Bullets? Head Space? Scale? Temperature? Chrono? I'm running out of guesses. The rifle shoots all loads well; however, it shoots some loads exceptionally well. I'm looking to get one of the exceptional loads to work more than one day in a row.

Assuming more failures, when I get back I'm going to try the node method at longer range but I'm still hoping to tame the velocity before I start investing time in long range trajectories....

If the first reply shows up...my apologies for the second!