At 500 meters (rams) I'm getting about 2-3' of vertical stringing with 56gr of Goex FFg behind the 335gr Lyman 378674 bullet in my High Wall 38-56. With smokeless loads in modern rounds (.223, .308, etc.), vertical stringing at long range is generally the result of a variation in the amount of powder. Is that also true when shooting fixed BPCR ammo? Thanks.

I've had BP trapdoor loads do this when the barrel gets hot. Enough so that cool down solves the situation. 

Dick

Is that 2 or 3 feet ? if so it's a lot of stringing hard to imagine variance in well thrown powder charge could make that much difference. Few grains up or down will cause stringing, probably less than 1/2 moa or 2 or 3 inches at 500 meters. See this in fixed black less than modern cartridges since most a loaded full not a lot of room for variance in charges.

Not being a bench shooter I judge my loads by vertical. Generally ignore windage variance as long as it's not extreme.  At 500 meters my 38/55 fixed black or smokeless 290 & 300 gr bullet will show vertical disbursement under 10 inches.

I account a lot of the 10 inches to stock shape not being optimal for rest shooting.  Gun rides back on a curved forearm with a slight reverse tip. If it hits the tip it throws the muzzle up.  Mostly I shoot it rest off the barrel just in front of the fore-stock. Not the best for horizontal though. How it rides is critical.

Sighting error can cause vertical problems too but a good aperture will resolve pretty close, 1/2 moa probably. If a post front sight any amount of vertical is not surprising as light changes, particularly on odd shaped targets like Rams

We shot our club 500 meter offhand Buffalo match yesterday, while spotting I saw a lot of low shots. Most accounted to the shooter not following through, dropping the muzzle too quick rather than rifle problems. Some of the light pistol powder loads are noted for vertical if the rifle is not mounted same each shot. We have several shooters using Unique in small charges, generally see a fair amount of vertical misses from them. Having said that guy that wins the match often uses light charges of pistol powder. He is a very good shot with absolutely consistent mount sequence

Boats

Thanks for the replies. Some good points. 
- I'll move my rest point from behind the snabble-knob on the forearm to just in front of the forearm to insure a consistent rest position. 
- Will pull the buttplate and check for tightness of the stock. I put in a Lee Shavers trigger/sear, so may need to be retightened. 
- Will also try 57gr and 58gr to see if that helps. 
- Primers (215) seem to be nice and tight and see no indication of leakage or being pushed out. 
The stinging is actually ~24". Shots were an inch or two under the belly and 4-6" above the back. 
Front sight is a Lyman globe with two inserts, a small, fine aperture and a post. Rear is a Lee Shavers Soule with Haley eye-cup, so should be good with sights. 
A consitent hold would help, and could use some help there. Need to find a lace-on cheek piece, as my straight stock without a cheek piece allows me to wonder around from shot-to-shot.

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        Joe.   

I think I would look to how the gun rides the bags first.

We talk a lot about ultimate accuracy not sure it's real relevant to a rifle like mine built for position shooting hand held. If the bullet is well cast and fits the bore. Case is minimally sized and full of black powder most good single shot BPC Rifle will hold 12 inches on the ram line. It is easy to miss rams though and elevation is the most common miss. 

I shoot at a lot of rams, offhand mostly under rules that require post front sight (CLA) or at a ram that's too big for my largest aperture at our club single shot matches, and missing over the back is my most  common miss.   

To prove the mechanical accuracy you might mount a scope and use a bulls eye target at 500.  Assra's full size 200 yard target is a good one to test a scoped rifle at ram distance.   

Boats

It sounds like you may have a Brown/Chester highwall.  If so, your stock will probably be OK because it has a through bolt but it wouldn't hurt to check.  It's the original walls that shoot loose because of the way the stock is held to the receiver.  You do mention small aperatures.  Don't be afraid to try a larger aperature both front and rear.  Having some light around the target actually helps center it up better. Try without the post because the heighth of the post can easily hide on the animal if they're dark. The hardest to center up for me are the turkeys.  Weird shape.
When shooting off sticks, try resting the barrel about 4" from the end of the muzzle.
If you try all of our recommendations, you're going to be busy for a while Bob

Ole Win is correct bigger the aperture the better generally. I use them for everything except when rules don't allow or can't get one big enough.

Post give me a fit when the animal is painted black.  But that's the game and have to play within the rules.   If a black post black target I settle off the target and swing into it. Still my scores black targets are never as good as when the critters are painted white. Don't shoot BPCS anymore but when I did I used the largest aperture I could get.  Since distances are longer with the same targets in BPCS vs CLA most times you can get one large enough to circle the animal leaving plenty of light. CLA requires post so apeture size is not an issue.

X Sticks were you support the rifle is critical, and most important is to put it same place every time.  Worth wrapping some tape around the barrel or some other index point to make sure all is uniform. I pull the barrel back until the forend hits the sticks then move the stick forward using my left 4 fingers as a gauge every shot. I shoot several rifles off sticks and use the same index on all.

On your cheek piece. One problem with Rams is the sight is cranked so much rifles with nice fitting 200 yard stocks don't fit worth a darn at 500.  It's good to be able to get a nice check weld but if you pay attention it's possible to be consistent even with a high cheek low comb.  NRA rules on the rifle don't give you as much latitude as truly needed to get the rifle stock right.  Best solution you never see, two step front sight allowing about the same cheek postion CP Then Turkey Ram.

BPCS is a bastard match anyway.  3/4 prone 1/4 offhand with a rifle not built for offhand. And way to powerful if it knocks over rams for good shooting on the other critters. On top of that spotter is worth 40 % of the score Shooter 60 %  I gave it up for Schuetzen. It's was a lot of fun and taught me a lot about single shot rifles though so no compaints on the time spent. 

Boats