Accuracy Variations from Magazine Feeding

First topic message reminder :


I've been blabbing a lot about first and last round from magazines how they affect groups. I was digging through a box and found an old RR target from my match 45 I shot for many years. I had re-built the pistol for the 3rd time and was testing Federal Match 185gr jswc. Magazine was reloaded after 5th round fed into chamber. 1st & 10th shot were the flyers. Just thought I'd share.
Jon

Greetings,

For those of you that own a "Random Rest", seek out a Dynamicist or someone well versed in Physics.

Have them examine the rest with a pistol installed.

Then listen to them question the repeatability of all the mechanisms involved.

The instructions that come with the "Random Rest" say to fire several shots to "settle" the gun.

Try to visualize what events can "unsettle" the gun.

Personally, I feel you can obtain repeatable results using a sandbag rest and a scope of about 7X.

Cheers,

Dave

fc60 wrote:Greetings,

For those of you that own a "Random Rest", seek out a Dynamicist or someone well versed in Physics.

Have them examine the rest with a pistol installed.

Then listen to them question the repeatability of all the mechanisms involved.

The instructions that come with the "Random Rest" say to fire several shots to "settle" the gun.

Try to visualize what events can "unsettle" the gun.

Personally, I feel you can obtain repeatable results using a sandbag rest and a scope of about 7X.

Cheers,

Dave

Dave I agree with you and it all depends on the bench you mount it on and your technique using the rest.  I do well enough with my modified HySkore rest and a 6X scope.  A lot less trouble then using my RR and the results and close enough to tell me what I want to know.  Don

I believe vertical dispersion is more likely than horizontal dispersion to be effected by variations in magazine follower tension as it may effect how the gun cycles and recoils. Another symptom, of vertical dispersion is powder position in the case, because of varying velocity

Unless your bullet seating depth, your powder selection and volume have bullet base resting on powder then powder case position matters. Given your normal load & a chronograph run your own test. Shoot five rounds  through the chrono in the following three manners:

1) Just shoot as you would. Try not to let what you read in 2 & 3 effect what  you would have done. Notice velocity of each shot fired, SD and AVG fps
2) Hold gun vertical, muzzle down, then carefully raise to horizontal not going above horizontal for each shot. Note velocity of each shot fired, SD and AVG fps
3) Hold gun vertical, muzzle up, then carefully lower to horizontal not going below horizontal for each shot. Note velocity of each shot, SD and AVG fps.

What you will find is that in all cases in #1, the first shot will be lowest fps or one of the lowest. The SD and will be the greatest. #2 will have the lowest AVG fps. #3 will have the highest AVG fps, the lowest SD.

This condition is most noticeable when shooting moderate to low power loads from a holster. In this case the first shot will be significantly lower than the others.

I bought some Zero 200 gr. SWC a couple of years ago and tested the only load I used back then.  This was my first and only test target I shot off sandbags.  4.0 gr BE, CCI LP primers, .469 taper crimp, seated per NSK recommendation of .928" base to shoulder.  I was curious about the cause(s) of the 3 flyers.  Was it my technique off sandbags, my reloading inconsistencies, or the bullet itself?  Now I must consider if it was the first/last rounds off the magazine. To my best recollection, there's a good chance a couple of those flyers were the 5th and 6th shots.

Let's take a look the a few of the bullets I picked out of one box. Since it was a small purchase, it was shipped via UPS ground and some or many of the dents around the base may have been caused by rough handling.  The bullet on the far right shows a couple of dimples on the side. Since these test 10 rounds were not sorted by appearance, I wondered how much of a dented base would cause the bullet to go off track. Any and all comments welcomed!  I only shoot lead.