Been reading of your problems with the GPR. I have a couple of things for ya to consider also:
IMHO, I think you need to sandbag the rifle in the front & the rear because you have not eliminated shooter error. And this could well be a major contributor to the problem.
Also I am wondering if you are shooting too much in a day possibly getting tired, upset with the rifle, over anxious, & seen this happen lots of times.
If you shoot a good group of 3, just go on.... Shoot another group of 3, then go on. All that matters is you get them to group right now, and you got all your life to see how consistant Mr.Nick can shoot a group of 5-10, right now you need the rifle to perform.
Another thing I would try is some 2F powder in it. Some rifles seem to like that better. I think you should find a accurate load between 70 & 90 grains in 2F or 3F.
Also I think you should swab between shots & unless I missed it, I see no mention of that, I suggest modifying a .50 cal cleaning jag & in a drill with a 3 corner file so the first land is about .470 & the other are about .475 and the edges tapered back away from the muzzle & sharp. Swab with a liquid and use a teeshirt patch. NOT a tight swab, just a barely snug one, swab down & back one time, no pumping or extra work on it, just one smooth motion. The amount of lube on every patch must be the same each time. Here is a photo of a modified jag. The jag being slightly smaller on the front will push past the cruds & then gather the tee shirt material with the sharp edges & pull some cruds out on the way out. Keep in mind we are keeping the barrel consistant, NOT cleaning it. Also I find old cut up tee shirt material works som much better than patches & cheaper too. Shooting patches are usually too thick & too stiff & puth the druds into the breech & cause more problems than they eliminate. IMHO, your results may vary.
I would try some Lehigh Lube in the rifle or a good liquid lube in the rifle. I find that all of my rifles shoot a tighter group with a liquid lube rather than a grease patch. Again, the amount of lube you use on every patch must be the same. I suggest a lil 2 oz pump/spray bottle (Walmart) and usually 2 sprays on a dry day & 1 on a wet/humid day. Depending on your climate you may have to go to 3 for swabbing & 2 for shooting, just depends on the area you are in. I am using 2 to swab & one to load most of the time in my GPR except in the dry July or August & may go 3 & 2 on that. But the lil pump/spray bottle is the way to go with the liquid lubes in my rifles as it is very consistant.
One other thing I will stress also & it was mentioned above, it is Most Important that you only change one varible at a time. If you change patch thickness, all of the loads you shot previously are void. It is just like changing the pitcher in a ball game...... You start all over with the sequences. Same way with changing lubes, powders, ball size or brand even tho they be Supposedly the same dia, try the lst one you used & itf it works, fine. If it don't group, go up 5 grains & test then go down 5 grains & test & if that don't work ya have to start over.
I shoot 3 groups of a combo, and I write exactly what I loaded on them, Ball, charge, type powder, patch thickness & material, lube & how much, etc. I gem clip those together & then go to the next 5 grains up & do it over. Then when I have tried all I am going to try, I lay them out on the table & annalize them & see what is best. Then the next time out I go back & I work that load & combo again & 5 grains up & down from it. Then I annalize it again & so on.
Sometimes it takes me 3-4 settings at the range to find the right one. Sometimes it may take me 10 setting to find the right one. I feel most of the time the problems incurred in doing this is me & inconsistantcy on my part & usually when I have a problem, it ends up that was the contributing factor to the problem. Most people have a tendency (including me) to blame it on the rifle & usually it is not the rifle, it is the shooter. That is why IMHO front & rear sandbags is essential til ya get the rifle right, eliminate that varible & then work on you.
And the last think I can think of is the possibility of having a barrel under stress in the stock. I have seen one now & then you have to really push down the barrel to get the wedge pins in & this can cause a heat deflection on a real hot day & lots of shooting. A lil tension is OK but allot is not & you must relieve the area to get the stress off the barrel.
When you find the right load & combo, that rifle should shoot 1" groups or unde that at 50 yards. IMHO
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