What powder are you using and in what caliber?
The regular Winchester large rifle Boxer primer has the reputation of giving more hot gasses and hot particulate, what I call flash, than the Magnum Boxer primer from some other manufacturers, and more flash than any of the other regular primers. More flash generally means more pressure, less flash generally means lower pressures.
More flash is not needed in many applications and might give problems. Benchrest shooters want enough flash to get things going, but no more, as too much flash can cause larger group sometimes. Then again, if the temperature is low and you are using some really hard to ignite powder, you might need the extra flash. Insufficient flash and you can get hangfires or duds.
I use a lot of WC 860, a super slow ball powder, in a 6.5x55 Swedish M96B, and it is the one powder and caliber I have seen that shows much smaller groups with hotter Boxer primers. Winchester used to make a Magnum rifle Boxer primer, and I get much smaller groups with it than the regular WLR.
I don't shoot Benchrest, and the only rifle caliber I believe I have seen better grouping from the mildest Boxer primer, is in the 7.5x55, and the difference did not seem that much.