Some serious artillery geeks showed up yesterday!
This *was* going to be the final clue...
"bgy57" and Heath got it correct - though Heath hedged his bets by choosing both options for base-boosted artillery rounds. BGY's only hedge was scale-related.
It is indeed the rocket motor for an artillery Rocket Assisted Projectile (RAP) - in this instance, 155mm. Here's a pic of a sectioned Brit version (generally the same as the US except for nomenclature, I believe).
And here's a closeup of that round, focused on the motor.
RAP rounds where developed to give artillery greater range. You fired them with max charge, the motor lit off, and the round went further than a standard round would with the same amount of powder and elevation. The trade-off was increased cost, reduced payload (amount of explosive) and accuracy. The rockets just didn't burn as predictably as you'd like. But you got some more range. Oh, and there was the sort of thing that annoyed people like Heartless Libertarian and CAPT H - if the rocket motor *failed* - whether by not igniting at all, or an incomplete/intermittent burn - it's impact point became pretty much random along the line of fire. Of course CAPT H and Murray are always asserting that artillery impacts are *always* random, but that's just the envy-fueled lunatic ravings of soldiers jealous that they have to actually *see* their targets to hit them, so we pretty much ignore that. Which is another gripe they have.
RAP didn't last that long in the inventory, as we discovered that "base-bleed" projectiles, which have a little pyrotechnic package in the base, gave us almost the same effect without the cost, complexity, accuracy issues and payload reduction associated with RAP. Rather than boost the shell, base-bleed reduces drag caused by the partial vacuum that exists right behind the base of the projectile - the pyro packet burns, producing combustion gases that fill the partial vacuum, thus reducing drag and gaining you an increase in range.
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