groups dropping advice

[rockwind1]

as i have been testing out some loads, once again today, 6 loads, 6 different targets at 225 yds. as i went left to right with the different loads, my groups kept dropping about an 1" for each new group of 5. by the time i did the 6th load, the group was 6" low.

i have a suppressor on the end of my remington pss 308 24" stock barrel. it it a pretty thick barrel, the only thing i can think is the barrel is drooping as the it gets hotter. the groups were pretty good and the barrel did not get super hot. i think it took me about 45 minutes to an hour to shoot the 30 rounds (5 per load) plus about 5 for some warm up zero shots.

i just find it hard to believe that after 30 shots, my barrel will shoot 6 inches low at 225 yds. does that seem normal?

in case i didn't describe it properly, imagine shooting groups but for each group you adjust your scope down 1/2 moa, only i am not adjusting anything.

 

[Terry]

What did your chronograph say?

 

[gambleone]

No, not the barrel. More info would be helpful, powder, grains, bullet, shooting off a bench?
Any wind?

 

[Dusty Stevens]

Were you going down in powder charge? Was your rifle getting in a bind on your rest from moving to a different angle for a different target? Was your scope drifting on you? Is your rifle properly bedded or is it still stock and settling in (action screws still torqued) scope rings/base getting loose?

 

[Bart B.]

Might be you held the rifle differently each time.

 

[rockwind1]

sorry for lack of info. yesterday, chrono said mv increasing as each group went lower. heavy mirage, light headwind, shooting prone with a atlas bipod on a midway shooting matt with a straightlaced rear bag. loading the atlas forward on the matt since there is a little flap to do that with. pretty rock solid. 12 3 shot groups going left to right. all went down in a geometric proggression.

today, zero mirage, almost no wind, nice and cool, beautiful shooting weather. same shooting style today my chrono ran out of battery, labradar so only got a few readings but after yesterday shooting some of the same loads, today i did go left right, shooting lightest loads first.

ironically, yesterday my "node" was at 43.9 grains. so today i did 43.8, 43.9, 44.0 gr 5 shot groups and 43.9 was not that good but 44.0 was really good. maybe it was my shooting

it is a nightforce atacr scope i have had for a while and i don't think it is drifting, i have put on a lot of scopes, it is in a spuhr mount, and i had a gunsmith pin the rail. it is rock solid. it is a remington pss that i have probaby 2000 rounds thru and i am just trying to do a load for it.

i am using imr 4064 with a 178 amax in once fired federal brass (fmgg) with br2 primers

any help appreciated.

 

[smoooth]

Cleaned your barrel recently?

If you have 2K rounds on this barrel, what have you been shooting in it until now?

 

[rockwind1]

lake city m118lr and federal fmgg. that's it.

i have cleaned it not too man rounds ago. what do you think?

 

[gambleone]

You said your chromo said the mv was increasing even though the group was dropping. Odd.
Headwind can drop your group along with the mirage could worsen the group, but this should just give you a bad group that would stay pretty much the same from group to group. Dropping an inch each group sure makes it sound like it's mechanical or the shooting style or a combination.
Check your scale for accuracy. Check action screws like Dusty mentioned. Try again & let us know.

 

[Bart B.]

Headwind can drop your group

A 20 mph headwind will cause that load to drop less than one tenth inch lower at 225 yards. Wind speed is about 29 fps but slows down the bullet about 7 fps at 225 yards.

 

[rockwind1]

there was zero wind today, and the groups dropped about 3/4" to 1" each time. and yesterday, when i said light headwind, it was 4 mph or less

 

[smoooth]

Did it group well as it was incrementally dropping off?

Personally, anytime I change powders or Bullets, I clean the barrel first and start fresh.

Also, please tell us about the suppressor, number of rounds thru it, direct thread or QD, brand, material, and if you have a mirage cover on it?

 

[tunered]

Is your first group allways on point.?

 

[rockwind1]

it is a silencerco harvester 30 caliber, a pretty common mid priced supressor. i mean, its no thunderbeast but it has shot well in the past on several other rifles, including this on, it is a direct thread and i do have a mirage cover on it. yes, my first groups are usually good as i have just zeroed the rifle. my first guess was that as the barrel warmed up, it "drooped" more and more as it got warmer, especiallyl with the suppressor torquing it down way out there on the end of the barrel.

i sort of thought when i posted here, that i would here that this is a common occurence. doesn't seem to be the case. so now i am think that as the barrel heats up the action, maybe it is expanding and pivoting on a hard spot in the stock. i could have sworn i had the action pillar bedded or something like that by a local gunsmith but maybe i didn't.

tomorrow weather looks great, i removed suppressor and put on a trustworthy jec brake i had. so will try that and see if it still happens. also have lots of the usuall lake city m118lr on hand so will try some groups with that also.

thanks again for any ideas

 

[Dusty Stevens]

keep us informed

 

[smoooth]

To help narrow down the problem, just change ammo and leave the suppressor on. Change one variable at a time. The way I am understanding this, based on the information provided, your reloads were the variable that changed and your groups began to drop off incrementally?

If you normally shoot suppressed with M118LR and this rifle + suppressor combination and don’t have this problem I would say the issue is in the reloads.

You mentioned your first groups are good, how were the groups as they dropped off? Pictures??

Did you check your suppressor for baffle strikes? Harvesters are Aluminum, could be all in the can?

 

[Texas10]

I'll be watching this closely. Had the same experience recently with 6.5 cm at 200 yds. Tossed the scope and put on a Sightron III. Now I'm starting to worry.....

 

[Bart B.]

Test without the suppressor on.

 

[dellet]

If you shoot a single group, of two from each charge as a first or final group, does the vertical spread match the pattern of drop in the groups?

This should help rule out temp other changes in the rifle.

A single group shot with bullets colored with different sharpies to define different loads might be helpful.

 

[rockwind1]

hi dellet, each group has it's charge weight, i am not sure on the vertical spread per group, since they were new load developement charges i assumed some groups would be not good and some would be good as i got close to the "node"

i am using a autotrickler, fx120i set up for powder so the loads are spot on, i have noticed that i really don't have any "flat spots" in the graph of mv increase with each .3 gr of powder.