velocities where slightly higher than what I got with WC872, but the lot of 872 was on the fast side, and the 5010 alittle on the slow side(as far as burn rate goes, not velocity). According to burn rate charts the 872 should be slower than the 5010 and H870. Overall the 5010 gave about 25-75 fps faster than the 872, with a larger charge.
WC 872
135gr 102gr max 3750fps
150gr 94gr max 3475fps
5010
135gr 106gr max 3820fps
150gr 102gr max 3530fps
AA8700
135gr 109gr just short of 3900fps no pressure signs
150gr 105gr max 3650fps (need to work with this load a little more)
H50BMG (No velocities for this one, except for 135gr with 106gr clocked 3720fps)
135gr I've taken this one up to 109gr and am going to try to take it to 113gr(no more room) No pressure signs yet!
150gr I only loaded up to 103 because I didn't think it would go any higher, but I tink I was wrong! Going to try it up to 105-109 or there abouts.
Basically all were pushed to the top (until clear ejector pin marks were vissible, and then backed off about 1.5gr av. (all were done in about 80 deg wether)
Of course you must also take into consideration the fact that this rifle has a very tight neck and 30"barrel and hits high pressure sooner than a standard neck will. But considering
I don't even have to resize the brass, just recap, recharge, seat the bullet, and it's done. I'll take the small loss in velocity.
Now for the 300rum with 26" tube, I think you probable could get comparable velocities with 150 and 180 grain projectiles respectively. (at least with the 5010 velocities I achieved, maybe a little slower!) I would think that 2900-3000+fps could be achieved with 240gr pills with 5010 (again, depending on chamber dimentions and barrel length) As for the .338 bores, I'm betting you could get close to 2750-2800fps with 26"tube and 300gr bullets with 5010. Again I would really think that tube length and chamber dimentions would make alot of difference here. A long barrel with long throat will allow you to load the bullets out further and increase case capacity, thus increase potential velocity.
I'm sure there is someone here that could tell you more definitly what to expect.
But a little more information would be helpfull, such as bullet weight and barrel length.