Concentricity and Group Size

[370bc]

For a hunting rifle that shoots MOA, what plus/minus should be maximum out of round? I just loaded up some 140 VLD's 7mm rem mag with new brass with bushing dies. I separated from .003 for the good, and all above went to another group. I used an RCBS case master for testing. This load will be used for deer and elk hunting.
Is there a formula that for every amount of out of round that translates into a group opening up by a certain percentage? Thanks.

 

[37Lincoln1]

Want some fun? Check the concentricity of factory loaded hunting ammo.

 

[Lapua40X]

... Is there a formula that for every amount of out of round that translates into a group opening up by a certain percentage? Thanks.

I'm sure some physicist could work up a formula for us but, based on all the reading I've done, there is no published formula we might find useful in making that determination. But for hunting purposes, where the shooter's personal target range limitations are commonly judged by the distance at which he/she can hit an 8 - 10 inch circle, I doubt the need for that degree of accuracy for reloading.
In addition to the very good suggestion made by 37Lincoln1, be mindful that the slightest change in pressure on the case by your finger or any other instrument you might choose, as you rotate the case over the surface of the anvil on the RCS Case Master, can dramatically change the readings on the dial indicator.

 

[amlevin]

... Is there a formula that for every amount of out of round that translates into a group opening up by a certain percentage? Thanks.

be mindful that the slightest change in pressure on the case by your finger or any other instrument you might choose, as you rotate the case over the surface of the anvil on the RCS Case Master, can dramatically change the readings on the dial indicator.

I've also learned that with the RCBS Case Master it's important that the case head be kept firmly against the stop all through the rotation. If it moves off, the taper of a case will cause a false runout indication. Ditto if the extractor rim is "boogered up". It also helps to check runout with a little case sizing lube left on the case. Less tendency of the case to "ride up" either side of the V-Blocks.

 

[fdshuster]

"Check the concintricity of factory loaded hunting ammo". I did just that using my Sinclair gauge. Average runout was .012". So much for the claim that factory ammo is "better" than carefully made handloads.

For my benchrest match applications: Maximum of .002" for match use. A maximum of .005" for first round fouling, basic scope sight-in, short range informal shooting. More than .005"? Would go into the scrap barrel if it did not straighten out after 2 to 3 firings.

 

[dkhunt14]

Why don't you take 5 of your worst run out shells and mark the bullet with a blue sharpie. Just the first 3/8 of an inch on the tip. Then take five of your best and shoot them at 200 or 300 yards off a bench and see how much it hurts. Shoot them round robin and the blue ones will show on the target so you can see which ones are best. Matt

 

[amlevin]

Why don't you take 5 of your worst run out shells and mark the bullet with a blue sharpie. Just the first 3/8 of an inch on the tip. Then take five of your best and shoot them at 200 or 300 yards off a bench and see how much it hurts. Shoot them round robin and the blue ones will show on the target so you can see which ones are best. Matt

Wouldn't it be funny if the ones with greater run-out were the best. Look at all the run-out gauges that might end up being scrappped 8) 8)

 

[bigedp51]

You can't make a silk purse from a sows ear.............

If your cases do not have equal case wall and neck thickness your runout will be out of the ball park.

I asked a neck thickness question here because of excessive runout and variations in neck wall thickness. I found out the cheaper blasting ammo for the AR15 is made from cases that are seconds or rejects and have large variations in case wall thickness.

Federal loads ammunition using Lake City cases that failed quality control inspection and didn't meet mil-spec military requirements.

As you can see below military 5.56 cases must have .002 or less neck wall thickness variation, some of my LC cases have .005 or more variations in thickness. ( Lake City rejects)

Bottom line, to get minimum runout and concentricity you need good brass with uniform case wall thickness.

Below I have modded my RCBS Case Mastering Gauge to keep the V-blocks from moving and bought the Hornady concentricity gauge for "bending" the cases with unequal case wall thickness. These poorly made cases are not worth doing "ANY" prep work to before shooting. Meaning they are just AR15 blasting/practice ammo for shooting at 100 yards or less.

 

[338 Mollett]

Like most I strive to make the best quality ammo I can,but I've said before you are doing well to make .308 ammo that out shoots federal gold medal match and the run out on that ammo is poop!

 

[Barlow]

Want some fun? Check the concentricity of factory loaded hunting ammo.

I did that one time at the local gunshop. I didn't spend hours at it, but checked the major brands a couple times each, Hornady was the clear winner that day, but I can't recall 2nd or 3rd etc. Maybe I'll do it again and post the results. Barlow

 

[Shynloco]

"Check the concintricity of factory loaded hunting ammo". I did just that using my Sinclair gauge. Average runout was .012". So much for the claim that factory ammo is "better" than carefully made handloads.

For my benchrest match applications: Maximum of .002" for match use. A maximum of .005" for first round fouling, basic scope sight-in, short range informal shooting. More than .005"? Would go into the scrap barrel if it did not straighten out after 2 to 3 firings.

Even not for competition use, but for accuracy expectations, fd gives you some decent parameters. Personally, if after two firing, followed each time by annealing, if mine are more than .003 out and do not correct to .002 and below, they're history or go to the fouler bin.