Question: During the bank heist, where did the three men's large weapons come from? They could have tucked them in their suits as they did while leaving after the robbery, but the weapons would have been very visible. Where did they come from?
Answer: They have the rifles slung over their shoulders underneath their jackets. You can see the impression if you look closely.
Answer: No, that's a mistake. A carbine style rifle with an 11.5" barrel still has too much length, even once the stock is collapsed. See imagine here: https://c5.staticflickr.com/2/1649/25492440284_c3ba0c97cf_k.jpg It's a long, heavy piece of machinery that would easily profile under a sport coat.
Answer: The CAR-15 type rifles the bank robbers have can be fitted with telescopic stocks. With the stock collapsed and a 11.5 inch barrel (which is what it looked like DeNiro had when he used his rifle to shoot through the windshield of the car he was in) it is relatively easy to conceal the rifle beneath a normal suit jacket. The only thing visible may have been the flash suppressor at the end of the barrel, but most people wouldn't recognize that anyway.
Mobrien316