I have a 300 win mag Browning A-bolt with a BOSS attachment. Browning beds the action of their BOSS rifles with a small dab of slightly rubbery material. I shoot a hand loaded cartridges of 165 gn SST bullets with 73 gn of 4831SC shooting at a velocity of 2947 fps (SD 23 fps). I didn't get very impressive results. I shot 5 groups of 3 rounds from a Caldwell lead sled rest. The vertical group size ranged from 0.5" to 2.2" at 100 yards. Due to the inconsistent grouping I decided to remove the Browning bedding material and properly bed the entire action. The rubbery material is supposed to dampen the harmonics and work in conjunction with the BOSS system. Hopefully the rifle improves instead of getting worse. I'll let you know how it goes in a week or so.
This was my first bedding job and I set the action and barrel a little too low in the stock. So I skim bedding it again and it turned out very well considering it was my first time. I took it to the range and shot from a lead sled to reduce any human error. I shot three groups of 3 rounds. My vertical spread at 100 yards was from 0.25" to 0.5" which is as good as can be expected for a factory rifle. Unfortunately my horizontal spread was from 1.2" to 2.2". I can't figure out exactly why I'm getting such a large horizontal spread. The wind was very mild. I checked my scope and it was secure and my action was solid into the bedding. The barrel is free floating with a large clearance. The only thing I can think of is a bad scope? Any ideas what could cause such a large variation in just the horizontal spread?
Is 2947fps your max load? My 300 liked to spit out 165's at 3020fps with H1000. This velocity worked well for my barrel. It could be that your velocity is setting up bad harmonics in your barrel. Try increasing your powder charge in small increments to see if groups tighten up.
On your scope, I too have had a scope's internal adjustments go haywire. Try mounting a different scope and try again to verify what the problem is.
On your bedding job, is the action stress free? Did you just snug action screws or torque them while bedding material was drying? Torquing action screws can make the action touch high spots in the stock and bend the action to fit the stock.
I don't know that you can improve on a properly tuned BOSS -by changing it's bedding.
Most people set the tuner to Browning's recommended(in the BOSS manual) number and work up loads as usual. Then, they can tweak the tuner from there.
Was that the best load you could put together?
A friend has an A-Bolt. It may be the stalker model. I noticed that the fore stock is very flimsy and may well be making contact with the barrel under some conditions. The trigger is also very heavy. He's not real serious about long range or accuracy. I would have to fix those two things on his rifle before wasting any more ammo. I'm not sure how that relates to A-Bolt rifles in general.
I also have the stainless stalker with a synthetic stock and noticed the same issues. My trigger is adjustable and one of the first things I did was replaced the spring in the trigger. It now has a nice crisp 1-1.5 lb pull. I also routed the stock out around the barrel when I bedded it to make sure it does not come into contact with the barrel, even when using a tripod. There is plenty of clearance around the barrel.