Effects of Bullet Stand-Off .....

[mehavey]

Some might have been following the saga of the (new) Model-70/`06 that wouldn't shoot straight since last December. I sent it back to Winchester. They sent it back to me "within spec."

So I pulled all the hot glue bedding out, pillar-bedded the receiver out to the end of the chamber area under the barrel (thoroughly floating the rest of the barrel), recrowned the muzzle to a standard radius shape, and had SG tune the trigger to 2-lbs. Even though the "sporter" profile barrel is still whippy, it now "behaves." That said, I went back to the bullet that started my Winter of discontent: The 168SMK.

QuickLoad told me that IMR4831 filled the case (and then some) for respectable velocities and also burned cleanly (98%) in a 24" barrel, so that's what I worked up last 10 days ago/Temp was 65degr.
168SMK/IMR4831/57.7gr/0.005" off the lands gave 3/4" groups at 2,725 fps.

Yesterday I loaded up the same recipe in four 5-round batches: 0.005", 0.010", 0.015", 0.020" off the lands and fired a ladder at four targets: One of each standoff distance at each target for four shots, then cycle through each target/standoff again over about two hours. (It was blowing/gusting up to 35mph so I tried to shoot quickly between the worst gusts. The finer effect on grouping was to be expected.) The Temp was 88degr.

First thing noticed was velocity for the same load was now 2830-ish. That got my attention as to the effects of temperature.

The second thing was the interesting pattern that developed when I overlaid all four targets on top of each other and plotted the impacts for each stand-off distance. That whippy barrel does have a sweet spot:



...and the group differences show not only dispersion, but muzzle windage offset upon bullet exit -- both above and below that sweet spot.

A Krieger is looking better and better.....

[geetarman]

Interesting. . .

What I have found on my .308 and 22-250 is there are usually two sweet spots.

One is the factory OAL and the other is about -.065 to -.080 off the lands.

I used the Berger method of loading into the lands and then backing off -.040, -.080, -.120 and standard OAL. Not every throat will allow you to do that but my rifles did.

Another method would be to load into the lands and measure the OAL.

Subtract the load manual OAL from that value for the bullet you are using and divide by 4.

Make up 10 rounds for each value off the lands and shoot them and plot.

In my case I found the groups converged between -.040 and -.080 inch.

I then split those distances up into 4 groups and made 10 more rounds for each distance off the lands.

I found the best loads between -.065 and -.080 in.

I have found the -.065 in. for the .308 and -.040 in. for the 22-250 to work best.

The factory OAL for each caliber is really not bad if you want to make sure the rounds will feed through the magazine. A little tweaking can almost always make it better.

Once you get the sweet spot for the bullet you are using, you can tweak the load and see if you can make it better. It may very well make it worse.

I have found that maximum loads do not work as well as moderate loads.

Geetarman

[mehavey]

Another method would be to load into the lands and measure the OAL.
Subtract the load manual OAL from that value for the bullet you are using and divide by 4.

Yup... In this case w/ that short-throated Model-70, the Distance-to-Lands was 3.307". Sierra's recommended OAL is 3.285" ...leaving me 22-thousandths to play with. (Hence the 5/10/15/20 choice.) Given the results, I'll probably go 13-thousands off (comparator-based) w/ that bullet w/ that load.

My challenge then comes down how much velocity variances (because of temperature) play out here. Finding a load that works at 65degrees/V1 may be give me a totally different result from that same load at a summer 95degrees/V2. My understanding is that match/benchrest shooters actually chronograph at the range the day of the match, then adjust/load powders right there to shoot for a specific velocity.