The Exlorator comes with a default "boltgun", but as it is described in the book, it is a smaller (or normal-human adapted) version of the regular bolter that the Adeptus Astartes use (which is called Locke pattern). I would like to know, how different are these weapons? Is a "bolter" the same thing as a "boltgun"? If the Astartes bolter has .75 caliber, what's the smaller (?) version's caliber?
I don't know the exact rulings of Rogue-Trader, but I do have the Deathwatch manual on hand so can speak about some of the differences between Astartes and non- boltguns as defined in that manual, which (to my knowledge) is supposed to be compatible with Rogue Trader.
Firstly, boltgun and bolter are the same thing: they fire "mass-reactive shells called bolts" (Deathwatch Core Rulebook P146). There's no mention of smaller-calibre ammunition for non-Astartes bolters, only that "most bolter models have their roots with the Space Marines, despite smaller, inferior adaptations treasured by Imperial officers and nobles" (same page). Astartes weapons also count as "one Class heavier to anyone lacking in Space Marine physiology", and "impose a -30 penalty to all Tests made with them by presumptuous mortals due to improper scale, weight, and/or recoil--even in power armour" (same page).
The section on ammunition in the Deathwatch Core Manual (P158) takes care to explain that "bullets from one firearm cannot be used in another unless they are similar in make. Autoguns and autopistols can usually interchange rounds, but they calibrated bullets fired in terrible volleys from Astartes assault cannons are unique to that weapon." There's no such warning for bolt shells however! Everything points to .75 calibre bolt shells being the default for a "standard bolt weapon", i.e. both Astartes bolters and shoddier bolters.
\$\begingroup\$ But it still makes not sense to me. If the weapon is smaller, but it shoots the same ordnance, shouldn't both weapons have the same recoil? I've read the recoil of a bolter is what makes it so hard to handle (after all is more like a mini-rocket launcher), so just making the weapon smaller doesn't solve this problem for mere mortals. \$\endgroup\$ Jan 28, 2014 at 23:07
\$\begingroup\$ I'll be honest, I wouldn't be surprised if Astartes bolters had intentionally more recoil: there's an extra mention in the Deathwatch rulebook for an optional DM rule that enforces extra penalties on 'unworthy' wielders of Astartes guns. Mere mortals are supposed to have problems with bolters anyway, so you're right in that making the weapon smaller doesn't solve everything -- it only makes the gun fit in puny hands better. \$\endgroup\$ Jan 29, 2014 at 0:46
\$\begingroup\$ @Golokopitenko The recoil depends on the amount of force with which the ordinance is propelled away from the firer, not the weight of that ordinance. A 1-gram needle could be propelled with the same force as a 1 kg cannonball; The needle would leave the muzzle at a much higher velocity than the ball, but both would generate the same amount of recoil. It's entirely possible to reduce recoil without substantially reducing calibre; When the rounds are 0.75 mm, 1000 m/s isn't that much worse then 2,000 m/s. \$\endgroup\$– GMJoeJan 29, 2014 at 3:25
\$\begingroup\$ @GMJoe - Bolters are .75 caliber, which is roughly a 19mm diameter round. On the other hand, though, bolters are essentially gyrojet weapons, so they could conceivably have a very low muzzle velocity, then accelerate after leaving the barrel. \$\endgroup\$ Jan 29, 2014 at 11:47
2\$\begingroup\$ Warning: given that Astartes technology is generally not well-understood in terms of our conventional sciences, I'd be wary about taking a discussion about the physics of bolters too far. \$\endgroup\$ Jan 29, 2014 at 18:21
Aside from the bits cited in ioanwigmore's answer, Deathwatch also assigns a higher damage value to Astartes bolters than to those made for normal humans. (Originally 2d10+5, later errataed to 1d10+9.) Since they both fire .75 caliber rounds, this implies that the Astartes bolters fire rounds which are either faster, denser, carry a larger explosive charge, or some combination of these three factors.
If the Astartes rounds have a higher muzzle velocity or higher mass at the same muzzle velocity, this would increase the weapon's recoil, explaining the increased difficulty for mere mortals to handle an Astartes bolter. It would also increase stresses on the weapon itself, making it dangerous (or impossible, if the cartridge is lengthened to allow for additional propellant) to use Astartes ammunition in a non-Astartes weapon.
Of course, that's all conjecture, given that this doesn't appear to have been directly addressed in canon.