3+2 group: what does it mean?

[Matthew T.]

I shot this at about 90 yards with my Ruger M77mII 6.5x55 using Hornady 95-grain V-Maxs in front of 48 grains of IMR 4350 with standard Federal primers and Remington brass. My first two groups looked like sub-groups of two and five, this was just the best one. These aren't really hot loads. Do you think the barrel is shifting after two rounds, or am I just very consistent in pulling my shots? I haven't really noticed this with heavier hunting bullets. Then again, this is probably the tightest group I have ever shot with this gun. Opinions?

 

[Dave P]

I have experienced similar with a not-full case of powder. Namely 3006. Different POI if the powder was in the back end vs the front end. Have you got any other powder that might get closer to 100 % fill?

Dave

 

[Matthew T.]

48 grains of IMR 4350 fills a 6.5x55 case almost to the start of the shoulder. According to Chuck Hawks, the case capacity is 55 grains of water.

 

[Shawnee]

Hi Matt...

How much time are you taking between shots?

What power is your scope set on?

The top group, if it is the last 3 of 5 shots, may be due to difference in barrel temperature - mirage after the second shot making your aimpoint look higher than it is.

 

[Pumpkinheaver]

Try letting the bbl cool a little longer between shots and see what happens.

 

[Matthew T.]

Time between shots: not much - between 20 to 30 seconds. I'm letting the barrel cool down until it's barely warm between strings (maybe 10 minutes).

The scope is a Burris Fullfield II 3-9 set at 9x.

 

[Shawnee]

Hi Matt...

20 or 30 seconds is too little time to wait. I've known some guys who insisted on waiting 10 minutes but I've found that 3 or 4 minutes is usually good enough to prevent the group from "walking" or "climbing".

HTH

 

[Matthew T.]

okay

I'll try four minutes between shots next time.

Since Rugers aren't free floated, are they more susceptible to having shots walk around the target as the barrel gets hot?

 

[Lambo119]

Hot barrel more than likely. But just to make sure shoot a couple of five shot groups and wait 3-5 minutes between shots. See what happens.

 

[Shawnee]

Hi Matt...

You asked... "Since Rugers aren't free floated, are they more susceptible to having shots walk around the target as the barrel gets hot?"

The purpose of free-floating a barrel is to eliminate any potential points of uneven pressure being exerted by the stock upon the barrel which, due to disruption of barrel vibration (not heat), would, or could, cause shots to "walk around the target". The opposite extreme - (fiber)glass bedding is to accomplish the same thing by virtue of the fiberglass molding itself to the barrel.
If anything, a barrel bedded (in wood or fiberglass) would theoretically cool quicker because the wood or glass would act to adsorb heat.

It is about a 95% chance that lengthening the time between shots will resolve the split-group scenario.

 

[Matthew T.]

thanks for the advice

I'll wait longer between shots next time I'm at the range and post the results.

 

[jlmurphy]

Wood or fiberglass would act as an insulator and prevent that covered section from cooling as quickly as the exposed area. Many varmint stocks have greater clearance around the barrel and even vents in the stock to aid cooling. When a barrel "walks " , it usually strings the shots in one direction, I had a barrel that would string shots until hot, and then would produce nice tight groups, exactly 3" from the cold POA. Uneven barrel heating or improper stress relieving of the metal in the barrel during manufacture is usually the cause. If your rifle has a contact point from the forend of the stock, it's vibration pattern will be constantly changing, and throwing the shots off. I would bed the action first, there are tips here in the forum, then free float the barrel. The double grouping could be a load problem, but I'd guess the barrel first.