Grouping in pairs

[wombat13]

I've noticed the following more than once with two different rifles: A group of four shots split into two pairs. The pairs are separated by about 1" (@ 100 yds) on a 30 degree angle and the shots within each pair are touching or nearly touching.

I think this means that the loads are accurate in my rifle and my shooting technique is the source of the problem. A THR member suggested that it might be rifle cant when I posted the question in the Handloading forum. Can rifle cant change POI by 1 MOA?

What else might be the cause?

 

[Tirod]

Four shot groups are too small a sample. Combined, you are seeing the real group size of eight shots. If any change occurred between them, such as reloading, add in the inaccuracy of a second cheek weld.

Shoot ten shot groups in one string, and you get statistically accurate groups. The diminishing return of averages works out to about ten shots to get a real idea of what the gun - and you - shoot. It's military standard to use ten shot groups to test and accept a firearm or lot of ammo. That was instituted in the 1950's.

The mathematics of averaging a shot group has been done for a long time. Some see the small group shot as indicative, but what a few bullets show is that the sights are still close after a long period of storage. Others claim only the first cold shot counts, fine, wait 15 minutes for each - but shoot ten, and you get a correct statistical sampling of it.

What those two groups show is deceiving, and is known as "chasing the zero." Don't be the guy doing that. Shoot ten shot groups to really prove what the gun does.

Post the best two or four shot groups on the innernet. Most do.

 

[wombat13]

Good point. I will definitely try shooting more shots in each group. What brought this on is that I'm working up loads for my .300WM. I really don't want to load 10 of each bullet/powder/charge weight combo if I can avoid it. My plan was to load fewer rounds of each combination for an initial test as I worked my way up the charge weight.

For example, I'm trying 3 different bullets with RL22. Given the min to max powder charges in the manual, I'm loading four different charge weights (70, 72, 74, and 76 gr). That means I would have to load and shoot 120 rounds just to do preliminary testing with RL22. Between work and family I can only get to the range about every other week and can only shoot about 40 rounds each trip (waiting between shots so barrel temp doesn't change too much). That means it would take 1.5 months just to do the preliminary testing with that one powder. If that's the only way, I guess that's what I'll do.

 

[Smokey Joe]

Putzy, putzy...

Wombat 13--Agree with Tirod--4 shots is nothing. You said,

That means I would have to load and shoot 120 rounds just to do preliminary testing with RL22.

Yup, at least that. It's called load development. If you chase the zero, with small-number groups, you will never catch it.

Sorry you can't get to the range oftener. Guess what--When you find the best of 70, 72, 74, and 76 grains, then you get to check it out further, in 0.2 grain increments.

It'll keep you off the streets and out of mischief.

 

[benzy2]

It's going to take a few rounds to get the best load. I would look up what others are having success with out of your model rifle. While each is it's own animal, you can typically find a few go to loads that you can start near without having to run the entire range of loads, powders, and bullets.

 

[kwelz]

Yeo I am with tirod. 10 shot groups for everything.
Yes it means you are going to spend a bit more time and ammo. but it also means it will be done right.

 

[Silent Sam]

I've noticed the following more than once with two different rifles: A group of four shots split into two pairs. The pairs are separated by about 1" (@ 100 yds) on a 30 degree angle and the shots within each pair are touching or nearly touching.

I think this means that the loads are accurate in my rifle and my shooting technique is the source of the problem. A THR member suggested that it might be rifle cant when I posted the question in the Handloading forum. Can rifle cant change POI by 1 MOA?

What else might be the cause?

It could also be a bedding issue. First shot POI group every time I go to the range is the most useful information in predicting where the next first shot will land. That group is what I use to refine my zero.

 

[helotaxi]

Look up the Optimal Charge Weight method of load development. You'll save a lot of money on components, a lot of wear on your barrel and a lot of bruising on your shoulder. Shooting a 10 shot group at every incremental charge weight is a waste. Finding a good load isn't initially about finding the smallest group, its about finding the range of charge weights that shoot to the same relative POI meaning that you've found the barrel harmonic range. From there, you can tailor the distance to the lands to shrink the group size.

 

[btaylor73]

I had the same problem and solved it with a better scope and it went away. Somehow it was the scope shifting due to recoil I guess.

 

[Smokey Joe]

Shifty 'scope...

Wombat 13--I think that I and several others have assumed this, but I'll go ahead and say it: Before you start to mess with trying different loads in order to achieve greater accuracy, you need to be sure that all screws are properly tightened, trigger and the rest of the rifle is all adjusted properly, everything is in good working order.

Silent Sam's comment abt. bedding brought this to mind. It's A LOT easier & faster to check all the screws for proper tension before starting load development. Also make sure that the wandering POI problem is not being caused by a problem within the 'scope. That will happen, and suddenly, with no warning, with an El Cheapo Mark 75 optical system.

So much for assumptions.

 

[WNTFW]

Post #8 by Helotaxi:
"Look up the Optimal Charge Weight method of load development."
Good advice! I found 2 good loads within 35 rounds. I even used the method at 50yds one time and got a good load that held up at 600yds. I am not saying they are fully refined loads But I found where to be in short order.

It could be a few things and like Smokey Joe I advocate checking the obvious first. Don't rule out shooter. I always treat myself as suspect. That said I did have a scope that the eyepiece would move a bit on it. The rifle would put 2 in the same hole then shift about 4" @ 50yds and put 2 in the same hole. It would move horizontaly usually and vertically sometimes. I had to hold the rifle so it would not move and move the eye piece to find the problem.

Another easy thing to check on a bolt gun is the mag box. Is it binding or does it have a bit of play in it?

"I've noticed the following more than once with two different rifles:" That would point to shooter unless the scope is the same. I shoot iron sights mostly and can put in some really tight groups. The flip side of that is I can blow out some poor groups. That is why I tend to go to shooter error for my own shooting. I have not put in enough shots to be consistently that good, but enough to know how easy it is to open up a group or throw flyers just by being ever so slightly lax in any apsect of shot execution.

 

[wombat13]

Thanks for all the info. The rifle+scope combinations in which I've noticed this paired grouping:

Ruger M77 Hawkeye .300WM with Leupold VX-II 3-9x40

CVA Accura with Leupold VX-II 3-9x40

I do believe that I am the cause of the problem, but I will double check all screws.

I will also look up optimal charge weight loading.

Thanks.