Vertical Stringing

[Northerntier1]

I'm new to BP and trying to understand all the variables. With my 38-55, at 100 yards I get 1 1/2 inch groups. When I move out to 150 yards I have a group an inch wide and 4-5 inches long. Shooting 252 grain 20-1 boolits off a solid bench rest. Does anyone have suggestions on tightening up the group? What am I doing wrong? Thanks for the help in advance.

[runfiverun]

that is usually an indication of velocity variations.
could be case neck tension,variance in powder compression,or ignition.
but what do i know i don't shoot b/p

[405]

What r5r said is pretty good guess as to what is going on. On the bright side a 1.5" 100 yd group is nothing to sneeze at and indicates a very good accuracy potential with your set up! Usually, horizontal dispersion is shooter induced or obvious wind drift and the like. Big ugly groups can be anything. Vertical dispersion (especially at distance... like 150 yards or farther) is usually velocity related. Beg, borrow or steal a chronograph. VERY handy in isolating problems and judging accuracy potential of loads.

BPCR is kinda like muzzleloader.... an art form. Variables to try can include: change wad or use a wad, less or more crimp, taper crimp, no crimp, change powder compression, change primer (for BPCR I like the least poop and have had good luck with Rem. large rifle), try seating bullet out to lightly engrave into lands. The list can get very long and the permutations of all the variables is staggering.

[JRW]

Of all the variables that are possible, make sure that you keep meticulus, accurate records and change one thing at a time. multiple changes at once do not really get you any closer to the results you seek.

[Northerntier1]

Thanks for the responses and encouragement. Back to the Bench. At least I know it is not something obvious so will just start one variable at a time. I did have a chrony and was running 1200 to 1250. Is that too much of a spread?

[runfiverun]

maybe try a diiferent type of target one you can see better

[Echo]

Boolits lose accuracy when they pass through the sound barrier (on the way down). At 1200 fps, your boolits are about just transonic when it comes out of the barrel, and therefore probably passes through the sound barrier before it hits the 100 yard target, but hasn't departed enough at that point for dispersion to show up grossly. By the time it gets to 150 yards the evil is done. Either slow down your boolits, or speed them up, if accuracy @ 150 yards is important.

[The Double D]

I don't know what rifle you are shooting, but that vertical stringing sound like problems I had with a couple of Rolling blocks. A slight change in velocity up or down would make it go away. Wasn't my idea, it was some one elses who suggested barrel harmonics might be the issue.

Anything else that affected harmonics might do it; bedding fore arm pressure, barrel band tensions, etc.

[Down South]

Could be a barrel temperature problem. Are the boolits shooting higher each shot? Are you letting the barrel cool back down to ambient temperature between shots? If the barrel is not free floating and as it heats up, it can create more pressure on the forearm from heat expansion causing the next shot to hit higher on the target.