Interesting Observation - Bullet Runout vs Case

[Steve Blair]

I'm holding all my match loads at .001 or less, but Im not sure why. Barlow

[br]
A fine shooter I know once told me, "There's about twenty things I do when reloading. I think only ten are necessary. I just don't know which ten."

 

[savageshooter86]

In addition, Tony Boyers book he mentions this same thing about runout. Some vendor was trying to sell them a tool to fix runout so he took the rounds that were reading up to 005 runout(vendor told him they would not shoot) and took them to the line and shot groups in the low teens. He also mentioned that he had rounds that read 001 runout too and they too
shot low teens(same load and shot same time as the ones with 005)

I just sold my concentricity tool as all these top shooters have said they don't see a difference on paper

 

[bigedp51]

There is more neck run out induced by the decapping rod/ball expander being locked down off center than any other cause.

Try a body die in combination with a Lee collet die or put a rubber o-ring under the decaping rod lock nut and let the decaping rod float.

NOTE, on semiautomatic rifles the rim and extractor groove of the case can get beat up badly and a ding/dent in this area can tilt the case in the shell holder on the down stroke of the ram causing more runout.

Also be careful with your case necks and your custom chamber you might have to turn the necks. Also full length resizing in body diameter and cartridge headspace length will help the bullet self center with the bore. Meaning give you cases a little breathing room in the chamber to counteract any case alignment error in your AR15.

I can't help be laugh every time I think about reading the statement made by a shooting buddy of German Salizar.

"I get my best accuracy when my cases fit the chamber like a rat turd in a violin case"

 

[savageshooter86]

Donovan thanks. I just thought it was interesting hearing a long range shooter and a short range shooter in two different styles of shooting share the same thought.

Main thing too is like mentioned above, is that a guy who shoots a couple matches a year will most likely not see any difference vs someone who shoots several thousand rounds a year and who practices a lot

 

[BoydAllen]

I would suggest that Tony was probably seating into the lands, with case neck thickness that limited neck bullet grip, and if he was shooting T powder, may not have been using a lot of neck tension as well. Combine that with the fact that virtually all custom FB bullets have a pressure ring that opens up case necks very slightly as the bullet is seated, and the fact that tight neck chambers leave limited room for maintaining misalignment as rounds are chambered, and you have a situation that probably only yields valid results for a limited number of the total possibilities of how ammunition and rifles may be configured. With thicker necks, more neck tension, more neck clearance, different bullets and seating off of the lands, you have an entirely different situation. Some years back, the NRA published a book on reloading in which one short article featured a graph that had been developed by the military that showed that ammunition that had more runout was less accurate. It has been a while since I looked at the article, so I have any of this wrong, please correct me.

 

[aj300mag]

Boyd, IIRC the Army test stated that for every .001" TIR >.002" equated to .25MOA enlargement in group size. There was no increase after .006" TIR.

My personal experience, I've shot low teen groups with .003" TIR loads. Boat tail bullets .02" off the lands pushed by V133, (.257" bushing).

 

[BoydAllen]

Good info. I typically have used a .257 bushing with 133 until I went to thinner necks, typically seat into the lands (although friends' recent experiences with Column bullets may have me testing .006 off for those), have not shot BT bullets much, and my loading equipment typically keeps runout on bullets at about .0015 or less, so I do not have any personal experience with the situation that you describe. Thanks for sharing it. Got to love those low 1s. Getting back to the military test, so between .002 and .006 there was a difference of 1 MOA?

 

[aj300mag]

Getting back to the military test, so between .002 and .006 there was a difference of 1 MOA?

I believe that was the final outcome of the experiment.

Did some quick and dirty load work up with the column bullets. Running them .005" off, pushed by 29.3gr of LT-32 shows real potential. I have to go to matches preloaded, hoping this will be the magic pixie dust load I'm seeking...

 

[BoydAllen]

I would preload some that are .3 grains lower as well, in case you run into higher ambient. I would guess that that is a pretty warm load, I might try the next node down to get a little flier insurance. Of course that is a situation that varies from rifle to rifle, and probably barrel to barrel. Let us know how it works out for you.
Boyd

 

[tonysnoo]

Hey guys, I've been reading/thinking about all this stuff and it's interesting. I'm curious Even after I have fire formed a brass two or three times there still is clearance in the chamber from the brass relaxing some after firing.

Just the effects of gravity(not to mention extractor pressure) on the case would offset the center line of the bullet to bore. I look at a reamer diagram. It looks like the dimension of my free bore is the tightest dimension in my chamber. Until it erodes shouldn't the free bore also help to align the runout some. Does it loose it's dimension quickly in the relative life of a barrel?

The other side of the question if the freebore erodes away and a round with run out is seated into the lands how would that help align it...center the nose of the bullet to the bore, yes, but what would it do for alignment?

Remember, I'm just a newbie asking questions.

 

[BoydAllen]

If you are looking at factory chambers, different calibers have different SAAMI throat designs, and dimensions, so they vary in their potential for straightening. The best that you can do is to have a close match between your ammunition and your chamber. This can be facilitated in several ways. The reamer that a chamber is cut with can be designed to create a chamber that is a closer fit. When a barrel is chambered, headspace can be set to a minimum value. Sizing dies can be ordered to minimally reduce fired brass so that it is a better fit a particular rifle, and seating depth can be set so that the bullet slightly engages the rifling. For target rifles that do not have to be able to chamber factory rounds, a tighter neck diameter may be specified, allowing control of the clearance between case necks and the surrounding chamber by selection of the thickness to which brass it turned.

Some time back, using a tight neck 6PPC, with about .0015 total clearance between a loaded round's neck and the chamber, with a round that was loaded to engage the rifling, I used a concentricity gauge, that allows straightening of loaded rounds, to move the bullet out of alignment to the extent that the runout of the round was .0035, instead of the usual value of under .002. I then chambered the round (at the range, with the rifle pointed at the back stop) removed it, and measured how straight it was. The reading was .0015. This is an easy experiment that you can try for yourself. Obviously it could be done with a dummy round, but in my case, the thought came while I was loading at the range, so I did it with a live round.