If this was a fight between A space marine and A guardsman, then it would be no contest. even a chapter vs an IG army of 10'000 men would not stand a chance, but as you pile on more IG then the odds get to the point where there insurmountable even for space marines. But then you have to factor in who sides with who special division wise. for instance the adeptus mechanicus are loyal to the imperium because they see the emperor as the incarnation of the ommisire ( Read:machine god)so they would probably fight along side the space marines if they could be convinced un-mass that the imperial guard were trying to harm or do harm to the spacemarines who are basicly the emperors children, so would probably side with them. same with the eccalistery and the sisters of battle. But its alot of ifs and buts. in a straight up fight between ALL spacemarines, so about a million all told who put aside grudges towards the other chapters, and the entire IG, so about one or three trillion on some mythical giant world where no one needs food or resupply of any kind, the space marines would fight but be crushed out right because NOTHING can stand up to somthing 3000 times the bodys it does. of course factoring in Titians if the adeptus mechanicus did fight that still does little to even the odds, again same with the sisters of battle. If we were talking about all of the non-standered imperium forces vs imperial guard that evens the odds a little more but not to the point where it would be anywhere near a fair fight. you would just be left with billions of dead and no clear victor becasue in that circumstance the gaurd would be reduced to such a pitiful amount it would not be able to do its job. so all in all 1V1 the spacemarines win 1V3000 Then its no contest the guard would win.
Also Astratys issue bolt rounds can take out a baneblade, if you dont belive me then read what the raven guard did to one in the SM codex with there attack bikes. ALSO not bias, and anyone who says other wize can answer to the box of spacmarines under my desk on the tabletop.
