The sheer bulk of the Astartes Bolter seems to be primarily reinforced housing for the mechanisms inside. The technological principles behind the boltgun are relatively simple, and given the reduced kickback due to the fact that they're smooth-bore, I doubt that the reinforcement is necessary for it to function effectively in that regard. Bolters of all stripes are big, and they're heavily armored because they're designed to take a lot of punishment. This is especially true when one looks at bolt weapons made for Astartes.
There is no doubt in my mind that Astartes boltguns are so heavily armored because they were intended to be used as a weapon every which way. Hell, they look like they could take a series of glancing blows from a chainsword and still function perfectly fine (machine spirits willing, of course). Those things are made for clubbing. Because they are made to be used by Space Marines.
No, I'm pretty sure they are big and reinforced because they fire giant bullets of doom and are meant to last for millenia through the punishment of war, through blocking chainswords and not being destroyed kn the random earthshaker barrage. If they were designed to be more effective at clubbing things than other rifle buttstocks, it would say so. For a weapon that IS designed to be used as a club, check out the ogryn ripper gun.
While they are certainly intended to last a long, long time, bolter parts are replaceable. Bolters are also smoothbore. The shells are ridged, and this ridging, when combined with the fact that bolt-shells are self-propelled, gives them the accuracy one would typically find in an assault rifle. The juddering of a bolt weapon would be bone-shattering if they were actually rifled like most other solid-projectile weapons.
When I said the boltgun was made for clubbing, I didn't mean for extended periods. I meant it was designed to withstand the stresses associated with being used by an astartes, including being used as a melee weapon should the situation arise. All astartes weapons are designed to take a ton of punishment, and most, if not all, can function as makeshift melee weapons, and are designed to withstand the stresses of being used in such a manner. If they weren't none of those relics would have survived.
In the novel Helsreach, Chaplain Grimaldus was swinging his plasma pistol around (bear with me, please) by its power-cable as though it was a flail. Plasma pistols are hard things to get your hands on, even for a well-established Chapter like the Black Templars. Nor are they something one would break, even if one's end was imminent. Grimaldus treated it as a weapon even after it had temporarily outlived its intended use as a ranged weapon, and it survived the battle.
My point is that one should not be particularly worried about using one's astartes ranged weapons as Improvised melee weapons for the purposes of attacking and parrying, taking the appropriate penalties for being Primitive and Unbalanced. And, of course, running the risk of being destroyed when doing so as is appropriate to the weapon that is being parried or blocked. (Giving chain weapons a 25-75% chance of destroying the improvised weapon in his situation seems appropriate, depending on the weapon's class, as it would indicate the astartes ranged weapon is perpendicular to the edge of the weapon, rather than striking the flat of the blade as one is ideally supposed to when parrying. Power Weapons should destroy the Improvised weapon uttlerly).
There's a subtle difference between the boltgun as I am arguing it, and the Ripper Gun. I am arguing that the boltgun is designed to withstand being used as a club, as a probable eventuality in the hands of an Astartes. The Ripper Gun is a club, having its own rules as a melee weapon seperate from an Improvised weapon.
Again, a bunch of these ultimately boils down to house rules :P