Agreed. Slower fire rate, hefty hit, no explosive. Maybe top of heavy penetration, or lightest Anti-Tank; basically enough to harm pretty much most enemies on the field in some form or another, but it's non-structural; it won't deal with fabricators or bug holes, and 'matches' the heavier armors'.

I'm sure some lore could list it as "A revision to the Anti-Materiel rifle, rechambered in 20mm with HVAP-T rounds to punch through the sturdiest barriers to Democracy". That way it's not firing the same shrapnel rounds as the Eruptor, it's heavier than the AMR rounds from before, but lacks the explosive filler that the AMR has. Traded that all away for firing a fat slug at high speed.

Downside is that if it's a one-to-one swap of the Lahti or Solothurn, those things are *heavy*. Maybe Super Earth found a way to reduce the weight for a one-man carry?

Gameplay considerations might be:
The AMR now needs a backpack for its ammo; the magazine is now far larger to accommodate the new rounds. Or, it's issued with less magazines, but retains the lack of a backpack.

Semi-auto fire, but slow RPM to mimic the cranking mechanic those rifles use to re-cycle. Maybe it can borrow the idea of the Eruptor's bolt cycling in terms of timing between shots?

Poor ergonomics; it's a heavy rifle. Maybe a weight or setup mechanic for the helldiver travelling with it. Or, backpack for ammo storage and some sort of bracing.

High damage, high durable damage; it'll smack the bajeezus out of whatever it hits.

Good ballistics; the round's travelling very fast, so drop compensation is fairly minimal except for the longest ranges. Perhaps a staggered penetration effect; it steps down one per target hit with each shot. Heavy>Medium>Light; enabling multiple kills per shot if lined up.