Load no longer groups, Nosler BT in .300WM

[wombat13]

Probably at least 6 years ago now, I developed a load for my Ruger Hawkeye in .300WM. It is a 180gr. Nosler Ballistic Tip with 76.5 gr or RL 22. (I realize that Nosler now shows 75 gr. of RL 22 as max, but Alliant still shows 77 gr. as max charge with a 180 gr. bullet). My records show this combination consistently grouping at 1 - 1.25" for four shots at 100 yards. Two years ago I developed a new load: 165gr. Barnes TSX with 72.8 gr of IMR 4831. This load is grouping 0.5 - 0.75" for four shots at 100 yards. This new load is what I've hunted and practiced with for the past two years.

Last week I went to the range and shot both loads in preparation for the hunting season. I shot two TSXs, three NBTs, and one more TSX. Last year I zeroed with the TSX and the NBT hit about 2" high. This year the TSX grouped as it always has, 0.75" on the bull, but the NBT was all over the place, in the four inch range.

Any ideas about why a formerly reliable load would no longer group? It's been a while since I cleaned the rifle, but the TSX is still grouping just fine. Is it possible that the NBT is more sensitive to copper residue? I'm happy to keep hunting with the TSX, but I'd like a second, cheaper load that I can practice with and hunt with, if possible.

 

[Nature Boy]

I had the same issue with 180g Nosler BTs in my .308. One of my best, most constant loads went from 0.5 MOA 5 shot groups to 3+ MOA in a rifle that continues to shoot everything else very accurately. I've yet to understand why.

I started this thread on this exact issue with that bullet

http://www.thehighroad.org/index.php?threads/ever-had-a-proven-load-turn-bad.797887/

 

[wombat13]

I had the same issue with 180g Nosler BTs in my .308. One of my best, most constant loads went from 0.5 MOA 5 shot groups to 3+ MOA in a rifle that continues to shoot everything else very accurately. I've yet to understand why.

I started this thread on this exact issue with that bullet

http://www.thehighroad.org/index.php?threads/ever-had-a-proven-load-turn-bad.797887/

Thanks for the link. Reading that thread has given me a few possibilities by thinking about what has changed in the years since I first worked up the Nosler BT load.

1. I replaced the trigger spring. The trigger is lighter and crisp now. This shouldn't negatively affect the group size of a particular load.
2. The barrel has worn in. I developed that load when my rifle was new. It has a few hundred more rounds down the pipe since.
3. I used JB Bore Paste. My rifle used to be near impossible to clean. I could spend hours cleaning it and a patch would still come out dirty. A few members here recommended JB Bore paste to smooth out machining imperfections that retain powder. It worked like a charm. Much, much easier to clean the rifle now.

So far, the most likely possibility is that bore of my rifle has changed and that is the cause of the change in accuracy. Maybe I have to work up a new load for the ballistic tips.

 

[horsey300]

Is this the same batch of powder? Makes me think of batch to batch pressure variances.

 

[Blue68f100]

If you have not cleaned the bore in a while, I would start there. Use the first 5 shot to foul the barrel before trying to get group size.

If different bullets, mic them to see of there the correct size. You may have some under sized bullets, not the first time this is happened. Same brass? #of firings? Has the brass been annealed? All these impact neck tension.

Are the shooting conditions near the same? Some times loads developed at 70°+F deg will not preform the same at 45°+F and vise versa. If you have a crony have you compared the results?

 

[GooseGestapo]

Could be lot to lot variation in the bullets. Could be worn throat in rifle, too.

 

[joed]

Get rid of the JB Bore paste, I took years off the life of a barrel with it.

 

[readyeddy]

Try Bore Tech Eliminator for copper removal. Best I've tried yet.

 

[Gtscotty]

I wonder if you might need to adjust your OAL? Between the bore paste and a few hundred rounds, your bullet jump might have lengthened out of whatever sweet spot you were in.

 

[Nature Boy]

I had the same issue as wombat and don't use JB Bore Paste.

I believe there's a lot to lot variation in these bullets, specifically the 180 BTs that's not apparent using normal measuring techniques

 

[wombat13]

Thanks for all of the replies. Here's my plan based on all of your input:

1. I don't use the JB Bore paste any more. I only used it briefly until the rifle cleaned easily. I won't start using it again.
2. I will clean the rifle well after hunting season. Thanks for the tip on Bore Tech Eliminator.
3. If cleaning the rifle doesn't solve the problem, I'll have to decide if I want to pull the bullets from the rounds I've loaded. Tedious job, but probably better than the wear and tear on rifle and brass from shooting them.
4. I'll weigh and mic the bullets that I pull. I can also weigh and mic bullets from a new box.
5. I have no way of checking lot numbers for powder since I didn't record it at the time. I will do so going forward for all my loads.