Your 300 grain Monoflex Hornady moves at 1950 feet per second at the muzzle as compared to the Partition Gold which has a 385 grain bullet going at 1785 at the muzzle. The difference in recoil I consider significant. If you are scoping this gun, consider eye relief.
I had the same gun as well as a Winchester 1200 and a Remington 870 slug gun, all with rifled barrels. I tried Federal sabots, Winchester sabots [both the supremes and golds], Remingtons sluggers and sabots, Federals sabots and all the Hornady sabot ammunition in each of these guns. Universally the Partition Gold stuff got the best tight 100 yard grouping but also had the worst recoil. I got equally radical grouping at 100 yards with both the Federal and Remington stuff so those left consideration. The Hornady performed well and had nearly as tight of groups at 100 yards as the Golds however when hunted in any kind of brush where small stems and stuff go unseen while aiming many times shots got deflected. That heavier, slightly slower beast of a bullet in the Golds simply pruned what was in the way. Where I hunt there are no established lanes that are clear of brush or limbs so sometimes things are there that just are not noticed when the trigger is pulled. I failed to notice a 5" red oak truck in front of a really nice doe while using the Golds and the bullet simply mowed the tree over before mowing the doe down, which dropped on the spot. The Golds stay straight in even some ugly brush. The Winchester Supremes would be my second choice hands down.
On the bench any of these bullets will have some butt moving recoil. As a seasoned shooter I'd suggest you get a box of each brand being considered and shoot at 25 yards only to see what kind of group they offer. Using the best two ammos for 25 yard groups I'd repeat at fifty or 100 yards using those two ammos to see which one held together best at the longer range one might find in actual hunting and then settle on that brand. Then work on scope/sight adjustment to clean things up. THEN I'd set the younger shooter down to shoot and have him wear the same clothes he might wear while actually hunting. When he's comfortable with the bench, have him shoot semi-off hand but using a vertical post for stability and then completely off hand....things change with each stance and he'll need to experience this change before hunting.
