What kind of rod are you using for the firing pin? Black or clear rods are less flexible so they don't bend nearly as much as a normal grey rod. That's my first recommendation. Secondly, check the placement of your bands. It's best to make attachment points on the exact same level as the barrel. If you can't do this, try to make attachment points on the top and the bottom of the barrel the same length away and hopefully they'll balance out. Thirdly, if it's your own gun you're building, I would recommend adding a pin guide. I don't really have an example on hand.
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kepp the bands straight in line with the pin, but if theres a pin guide, theres not going to be much of a problem
It's your rubberband placement. It should be centered and level with the pin, not up or down. If you can't fix that, add a pin guide.
Something is putting more upward pressure than downward pressure. Look to the answer in that direction.
Both Burf and NachoMahma are telling you opposite sides of the same problem. If it is a normal firearm... you most likely have a worn firing pin guide(the hole the pin rides in). When the SECOND one bent, you should have taken it to a gunsmith, who could have told you in about 30 seconds, and properly fixed it for you. In a crude explanation, when the hole wears out of true, or too big,it allows the pin to wiggle. Causing misalignment, jams, etc. It is also possible that the SEAT of the pin has worn, in which case, the pin isn't stopped where it should be. Like trying to drive a 3" nail through a 2" board, backed by a steel plate. It'll go fine, till you hit that steel, then bend. Both situations have the same fix. Either the firing pin guide hole will be bushed, and reamed to correct the condition. Extremely worn holes may require plugging and re-drilling. More expensive, and labor intensive, but cheaper than buying a new frame. As a final thought... if you replaced the pin yourself... you might have forgotten to trim it to length correctly. Even a few MM too long will cause a bend in short order. And each gun, although functionally identicle, will have worn differently, and parts need to fit precisely.
The pins are too small in diameter for the length required.
. Your hammer is probably striking it at an angle. Adjust the hammer to strike the pin on-axis (as much as possible).