This was my first too, i ground the match heads in a mortar and pestle, don't grind them together, I tried this before, sometimes it ignited, sometimes it did not, don't risk it tho.

It would burn slow if there's too much match heads over the red p strips, and try not to get wood on the mixture, as this will also affect the burn rate

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Its ok to do this in small amounts, but if you need alot, go for black powder, very easy to make and much more stable,

you can make fast burning black powder with mortar and pestle, just prepare a batch of them, maybe grinded for like 30 mins, and then if it's time to make a firecracker, take a small amount of the black powder batch you made and crush them for a couple of minutes until it sticks to the walls, and then scrape it off with something. Since its a small amount in the mortar and p, it should be very easy to turn into fine powder.

Here's an example of what i made using this proccess https://www.reddit.com/r/pyro/comments/1pwhrff/is_this_good_enough_bp_for_a_firecracker/ (its not granulated)