Runout vs group size

[wildcat284]

Hey guys,

Is there anyway to work out how big your group will be if your loads are showing runout? I guess what I'm trying to ask is if you loaded one round and it had 0 runout, then the next round had a runout of say .004, could you work out how far from the previous round fired this one was going to print on the paper.

Sorry if this seems a silly question, but today I was back out at the range, I shot nearly the perfect group out of my rifle. the second shot was touching the first, the third was through the same hole as the second, the 4th was touching the second and third, but the 5th was about 1/4 of an inch above the 4th. Some benchrest shooters commented that I should increase my charge by 0.2 of a grain, as vertical is a sign I need more powder.

I'm close to the ultimate load. I know I should probably be happy with a load that shoots 1/2 MOA, I can't help striving to get the ultimate load that is close to 1/4 MOA. I just love an accurate rifle.

All advice is greatly appreciated.

Michael.

 

[Hanic]

Hey Wildcat,

I'm just getting into the TIR testing also. I have loaded up a bunch of loads and segregated them by TIR for testing as soon as my schedule and decent weather can coincide. What I've found so far is that TIR under 0.004 at 100 yds doesn't seem to matter, at least not the way I can shoot. Out at 300 yds I have gotten, a very few) good groups with TIR of 0.001 or less. This is what I need to do more of, I just got into this as the weather started to get winter-like last year. More shooting, a good thing) is required.

But to try to answer your Q, I think the only way to understand how TIR effects group size in your gun, is to shoot different TIR in your gun and keep good records. Hanic

 

[fdshuster]

I also check loaded round run-out, using both the Sinclair and RCBS dial indicators. My cut-off is .004": anything over that is kept for 100 yd., I've gotten some great groups @ 100 yd with as much as .006"), but at 200 yd. and beyond there is an increase in group size. Cases with more than .004" are marked, and if they do not straighten-out after 2 or 3 firings, they are scrapped. The inside neck expanding button seems to cause the worst run-out problems, and I avoid using them when possible. If forced to use the button, the floating carbide button, available with the Redding, and now RCBS dies do the least damage/ self-aligning? I've checked some factory rounds and found some to have run-outs of as much as .012"! So much for the theory of "factory" being better than carefully made reloads.

 

[wildcat284]

My Redding Competition 's' type neck die set showed up last Wednesday. I resized my fired brass last night without the expander button installed on the decapping rod.

Fired brass in the chamber of my Kimber exhibits between .000 and .001 with the odd case showing .002. When I checked the neck runout on my sized brass, it also exhibited .000 to .001 runout. I have experienced times before when the expander button has introduced runout in an otherwise perfect case.

My bushing is titanium nitride, and just so easy to neck size my cases with, glad I went this route instead of a steel bushing

I have used the expander button in the past but from now on in these dies I am going to leave it off, except when any case mouths get badly dinged.

I'm looking forward to getting to the range next week to try these new improved loads.

Michael.

 

[mikecr]

Doesn't make sense that your runout goes DOWN with sizing, nor that you would see .002 fresh out of the chamber. Seems backwards.

Usually there is little to no runout hot out of the chamber. Then runout is brought out by sizing. The more sizing, the higher the resultant runout.

Thats normal.

 

[fdshuster]

mikecr: I'd expect to see these slight differences in run-out between "as fired in the chamber", and after sizing without the neck expanding button. All that tells me is the chamber is cut very close to perfect, and the sizing die body is also very close to perfect: the best of both worlds. The very slight differences can be attributed to variances in brass: some will have more "spring back" than others, due to thickness variations, hardness, etc. Even the best brass that I use,Lapua) will have these slight variances.

 

[yat3b35]

Wildcat284 you said that most of your bullets are touching except for one? That in my opinion is telling me that your load might be a bit to hot. In my 6ppc I had the same problem, spitting one round out of a group with all most every group I fired. My load was pushing toward the max. Talking with my smith we decided to lower the powder charge by a 1/10 of a grain at a time to see what happened. I finally got the best group with no spitters when I lowered the powder charge by 3/10th of a grain. The group became even tighter and no spitters. I had the same problem with my 6.5-284 but it took almost a full gr of powder to correct. Now if your load is not on the upper range of powder,hot) then maybe by uping your charge a 1/10 or so at a time will help.

 

[Clark]

Per the NRA and US Army in the A A Abbatiello 1960 article "Gauging bullet tilt", match ammo is most likely to be .002", while service ammo is likely to be .0025".
For a 1 in 10" twist 30-06, the math and the experiment agree .001" causing .55" extra group size continuing to .004" causing 2.2" extra group size. Beyond that, the chamber bends the cartridge straight.

OMHO, .262" neck 6mmPPC chamber is going to bend the cartridge straight long before it has .004" eccentricity.

 

[vinconco]

I'm finding that case wall concentricity is a major player. I use a NECO gauge and have confirmed on target that cases with .001 or less wall thickness variation perform better than rounds with low TIR and wall thickness variations up to .005.