Firstly, guns are banned in the UK, this is the best you'll get without buying one on the black market. Secondly, although the calibre is dreadful, this will still be classed as a live gun and will get the same punishment as if you had a real 9mm/AK47 etc, so before doing this, please choose what's best for you. I will try to get picks on within the next few weeks.
Okay, first you're going to need a blank firing gun, which one you get is up to you, if you can some how get a czech/turkish gun which are front firing this will save you some leg work and will have a better finish. Some 8mm blank bullets (these are easily available on the internet.) a quite a few candles, the more/cheaper the better, air rifle pellets .22's will be good or 6mm BB pellets. A vice (clamp) with a drill capable of 7mm/8mm diamatre
Now what you have this, you'll want to strip your gun, take the slider off, and clamp it and align ready for your drillbit, I've heard putting it in the oven can expand the 'barrel'but I wouldn't risk it. If you're going to do this, use some alliminium tape to dispense the heat. Now start drilling, get the speed up rather fast, you get the gist, just drill the metal in the barrel. Now the barrel is clear, we're not going to rifle the barrel, so be careful, if you push the gun powder/pellet it may blow up LOL. Also, the UK guns are zink and won't survive large calibre/combution so when doing this. tie some string to the trigger first to test it.
Get a knife, a kitchen knife would work, and get the blank 8mm bullet and cut the top (wax/plastic) whatever the fuk it is. now the top is clear, you have 2 options, get more gun powder from another bullet and add more to the bullet or just add the pellet, make sure the pointy bit is facing towards the opening, light up a candle until it melts and pour some wax over the top to seal it, make sure it's flush, and let it dry, now you're done load up and shoot away, this WON'T kill anyone, it may pierce someone's skin, but I doubt it will kill them unless you're at point blank range aiming towards the temple.
It's the best you'll get without buying a real gun, stay safe and have fun