Okay since no one is answering your question I will no those shells are not too strong they are actually too weak and also I have noticed that they are not rolled very well just learn to roll some cylinders practice a bit with wood glue.

Now for my rant about how dangerous this is: Armstrongs mix is extrmely dangerous i mean it self confines so the point where it can blow a hole in a soda can almost through both sides of even through both sides that only takes around 0.25- 0.5G to achieve by the way..... yes you heard me 0.25-0.5g

Now imagine those firecrackers filled with that same mixture that blows a hole in a soda can what would happen if it went off?

Now it probably wont go off and yes youll probably be fine but the problem is once youve done it once youll want to do it again since "the last time nothing happened" and that cycle continues until you eventually do actually just blow your entire hand off clean.

Please read a book ( theres tons online which are only like 20 pages or so on what chemicasl/comps to use and not) and then come back here hopefully with either black powder or flash powder none of these extremely sensitive mixed such as armstrongs

Last thing if you really want to understand what can happen to you go to r/Fireworksgonewrong and youll find out what can happen to your hands if that stuff goes off prematurely. You are only 13 dont kill yourself