Ah... exploding barrels. Never has something seemed so logical, yet been so rediculously improbable.
Seriously, go get a steel barrel full of gasoline, take it out to a field somewhere, and pump some shotgun rounds into it. It WILL NOT blow up. I promise you. I've done it.
Now, don't light a cigarette near it while you're examining it, tho... that'll doom you with those open flames near the fumes.
As for guns and armor. Have you guys ever heard the term "Bullet-proof"? I'm sure you have. But nobody seems to know what it originally meant. It was a test that was performed on plate mail. You take gun, fire a bullet at the breastplate of the armor at point-blank range. If it goes through, you screwed up on the plate mail. If it makes a small dent but otherwise leaves the armor intact, it's considered to be "proven" that the armor can take a bullet. And the little dent was called a "Bullet-Proof".
Hell, Knights carried pistols around on the side of their horse saddle, (a "side-arm"), to shoot at people while riding. After all it's much more effective to point and shoot with a gun then to try and use a long-bow.
Speaking of which: Longbows should be more powerful than crossbows and guns. After all, if you really look into the history, it was not the gun which ended the time of the knights, but the english long-bow. you can't make armor strong enough to shrug off the weight behind those arrows, so they were truly devastating to the clunkilly armored knights in a way light, soft, lead bullets could never be.