Rifle groups under .5in then opens to 1.5 after 10

[Rory S Sallee]

Dont know why but I can shoot two 5 round groups under .5min and after that it opens up to 1.5min. .308Factory Hornady match A-Max in a McMillan Heavy palma contour and doesnt seem to get too warm. Had a round I attempted to fire twice and not a very good primer strike, never did go off. Also chrono gave me a SD of 39 and I knocked out the high and low from the 10 round string and the SD went to 10. Could it be inconsistant primer strikes, bedding issue, something hitting inside the stock. Checked the float on the barrel and it seems fine. Tried cleaning didnt change, but cooled it goes back to normal. This is a work gun so I need it to run. Any input is greatly appreciated.

Thank you:

 

[Jerkface11]

Re: Rifle groups under .5in then opens to 1.5 after 10

Um exactly what kind of work are you doing that you need sub moa groups with a hot barrel?

 

[KeithR41]

Re: Rifle groups under .5in then opens to 1.5 after 10

That's a good question.

Sounds like you may have an ammo issue. Try switching primers. Maybe shoot some FGMM for a baseline.

41

 

[Rory S Sallee]

Re: Rifle groups under .5in then opens to 1.5 after 10

For the first response I compete for the agency I work for and I have to use the ammo they provide. Trust me I would much rather tinker on it and work my own loads. I dont think its the rounds as the other guns we have dont run into this problem.

Thank you:

 

[LRS_Ranger]

Re: Rifle groups under .5in then opens to 1.5 after 10

The only thing I can think of is heat. You have to remember that the barrel doesn't heat up uniformly, and after a fast string the bore is a lot hotter than the outside of the barrel. The fact that after it cools, it's right back to normal points to heat being the culprit.

 

[Captain Kirk]

Re: Rifle groups under .5in then opens to 1.5 after 10

How fast are you shooting your strings? You can shoot 10 rounds and then groups open up, but return to normal after you let the barrel cool, correct? .5 moa to 1.5 moa is a big jump.

I would guess its a bedding issue that drastically changes as the barrel/action heat up. You may want to take a look at the action to stock fit, maybe play around with lessening/varying the torque on the action screws, thats my best attempt at "arm chair gun-smithing diagnostics".

Kirk R

 

[rezmedic54]

Re: Rifle groups under .5in then opens to 1.5 after 10

What kind of rifle?? Rem, Savage, Win. meed to know

 

[Pusher591]

Re: Rifle groups under .5in then opens to 1.5 after 10

Not a smith but seems like it may be heating up and making contact somewhere down the barrel channel maybe?

I don't know just thinking out loud.

 

[jbell]

Re: Rifle groups under .5in then opens to 1.5 after 10

I agree, we need more info on the rifle. Also any mods that have been made to it will be helpful.

Just to clairfy, the accuracy goes back to .50 moa when it cools back down? Are you cleaning the barrel to get the accuracy back, if so how do you clean it and what solvents are you cleaning it with?

 

[UNCIVILIZED]

Re: Rifle groups under .5in then opens to 1.5 after 10

Dare I ask how much free float clearance you have? My thinking's that more is better, say 1/8" or so. May or may not help barrel cooling, but one likely wouldn't often fret over a bit of grit getting stuck between the barrel & stock that way. Nor about bipod loading flexing the stock to where it touches the barrel.

Does the groups opening up thing occur on the bench, when slung up/positional, both, or other? Ditto on whether or not it transpires under different atmospherics [read ambient temps here]? Also, what kind of stock? I ask as in my experience, some brands don't have near the forend stiffness of other makes. Such became real apparent when "playing" with slinging up one day LONG ago.

The more info & detail we get, odds are the easier it'll be to assist in helping fix things. And I know issues like you describe make me a touch testy, so I get the frustration bit.

 

[arborpro]

Re: Rifle groups under .5in then opens to 1.5 after 10

What distance are you shooting at 100? I don't believe heat would affect your group that much with that many rounds even with that poor SD. I shot plenty of high-power rifle competitions and I expect my rifle to hold minute or less at each stage and all those strings are shot at 20 rounds at each stage of fire. Don’t get me wrong I don't expect to shoot cleans at each stage but I do expect that my rifle would shoot sub minute if shot from a bench after each string of fire especially at 100 yrds.
If I were to guess I think your inconsistency's are based on your fundamentals. Natural point of aim trigger squeeze and follow through.
Are you shooting off a sling, or off a bi pod/or rest. If your barrel is floated properly you should be able to slide a business card all the way to the receiver. That would be good enough if you are shooting from a bench or bi pod. If shooting off a sling it should be floated so that piece of paper folded a few times can slide down due to the amount of side force from the sling pulling on the stock.
During rapid fire shooting as it appears you may be doing. Natural point of aim is key. You may be pushing or muscling the gun on target and slapping the trigger.
Is this gun/barrel new or how many rounds does it have through it. Use a good copper cleaner and try again. Dry fire several times on a target an inch or so in size (100 yards) watch what your cross hairs to so if they move. If they don't, try shooting a group again. While dry fire before each shot, be mindful of the wind. And lastly let someone else who is a good shooter try a group.

 

[Jerkface11]

Re: Rifle groups under .5in then opens to 1.5 after 10

Check your free floating like suggested. Just make sure the gun is good and hot when you check it.

 

[Rory S Sallee]

Re: Rifle groups under .5in then opens to 1.5 after 10

I keep falling back on the bedding as well and will have to take a look. Some where along the line the rifle was re-barreled and probably should have been looked at then, if it wasnt.

Thanks,

 

[Rory S Sallee]

Re: Rifle groups under .5in then opens to 1.5 after 10

Older McMillan 700 type action

 

[Rory S Sallee]

Re: Rifle groups under .5in then opens to 1.5 after 10

Good float on barrel, bagged and prone, and the stock is the M40 clone from McMillan in 700 type action. I also shoot F/TR and dont load the gun, free recoil.
Still perplexed on the SD being 39 on a string of 10, knocking out the high and low bringing it down to an acceptable 10. Fireing pin?? harmonic imbalance maybe. Combination of harmonic issues caused by fireing pin spring fatigue, in conjunction with poor bedding due to a re-barrel.

Thanks,

 

[Rory S Sallee]

Re: Rifle groups under .5in then opens to 1.5 after 10

I do all that and compete quite a bit outside work. Will go back through and check again. Had another shooter fire a sub half and when warmed up did the same thing to him as well. Tried the cleaning thing with same results. Govt. issued gun and came from anoter place with no data, or book. Shooting from 100 to set up new data, and shooting from bipod and bag. Trigger sucks I know that much but shouldnt effect 3rd group in string and not first and second.

Thanks,