rklapp,just to reinforce what other have said,that bullet is not going to shoot accurately in your Mosin no matter what powder or charge weight you use.
That particular bullet even if gas checked simply doesn't have enough dia. to engage the rifling properly in a .312" groove bore,which can cause gas blowby around the bullet which affects both velocity as well as accuracy. If that bullet were not coated it would have leaded your bore pretty badly within a few shots as the blowby would have burned the lube right off and left you shooting naked lead down the bore.
Undersized bullets also cause instability problems I'm surprised you weren't getting keyholes on your target but you can tell the bullet became unstable due to the large groups you were getting.
The plain base style bullet doesn't help much either although if it were of the proper dia. you could get some usable velocity and accuracy out of it but not jacketed bullet type velocity. With the Mosin's 1:10 twist bore and that bullet weight / length if it were of the proper dia. an gas checked your probably looking at about 1900+ fps. tops before you over spin the bullets as the bullets imperfection become more accentuated after you pass a certain velocity / RPM threshold with a given twist bore. The only way to increase that same cast bullets velocity and retain accuracy within the bullets usable RPM range is to use a slower twist barrel like a 1:12 or 1:14 but we have to use what we have.
In rifles when using the fast to med. fast burning powders that don't require a filler and ignite easily with reduced charges that give quick acceleration of the cast bullet,usable velocity / accuracy is going to be lower. Think of how the bullet is pushed into the leads on the one hand you pull up to a cars bumper and give it a gentle push to get it moving no damage is done to the bumper of the car being push. On the other hand ram that same cars bumper with yours at high speed and what happens to the cars bumper,the same thing happens to cast bullet along the entire length from the nose to the base,it get jammed,compressed and twisted by the fast acceleration induced by the powder burn. Slow powders like H-4895 give the bullet a slow gentle push an can safely be reduced down to 60% or the original MAX load and worked up to find the best velocity / accuracy with a given cast bullet. In general I've found that the slower powders tend to give better consistent shot to shot performance with a Dacron filler between the powder column and bullets base.
There is a reason you don't see RL#15 or Varget listed for cast bullets loads in cast bullet manuals it's not an appropriate powder for the cartridge / bullet listed. Reducing the charge weight of a given powder below what is listed or just on a whim that it might work is not only dangerous but will usually give you erratic results usually in the form of irregular burn / ignition characteristics which cause pressure spikes,velocity swings and poor accuracy. A chrono would have told you much about the consistency or lack there of,of those loads you were shooting.
Here are some of the results you can get when the proper powders and bullet combinations when the correct dia. bullet is used. These two loads were fired in my 1933 Tula 91/30 using a Lee .312" 185 gr. gas check bullet cast from clip on wheel weight plus some Tin and water quenched straight from the mold then sized and checked an lubed with Lars 2500+ lube a couple days later. The bullet actually cast right at .314" an are sized and checked as such. My two Mosin rifles have .312" groove bores.
Tomorrow will be day 2 shooting some test loads in my quest to get as close to the 2K velocity / RPM threshold with this particular bullet load combo. I'm up to just over 1700 fps. with 33.0 grs. of H-4895. I also have a good general purpose load using 19.0 grs. of Alliant 2400. But accuracy start to taper off as the charge weight goes up so I stopped where I was with that combo.
Two 5 shot 50 yd. test groups.iron sights.