Maybe an assumption too far. The bottom foot or so of JD's front door
was stained with blood. If the killer shot her with the gun in his left hand while holding her around the neck, shoulders etc with his right, then there would likely be blood over his right sleeve, as well as over the door. If it were BG, he'd had 18 months to get the coat dry cleaned a few times, so I'd be surprised if there was anything still to be found.
Well, apart from the photograph of him posing with a gun, that is. He denied it was him in the photograph, but his distinctive eyebrows can be seen, the gun disappeared, and he did not IIRC have any explanation for how photos of someone else posing in his flat with a gun came to be on his own camera.
Again, the source makes what I'd suggest is an over-confident assertion, or perhaps assumes its own conclusion. It's not clear he needed to have modified any gun, is it? He had clearly obtained
a gun from somewhere because he was photographed with one. That one would have done. At most he may have modified a bullet by opening it, reducing the explosive inside, then crimping it shut with pliers.
He had done 18 months in prison for attempted rape; prisons can be universities of crime, so it's conceivable he learnt how to do this in prison. Or indeed, just from reading around. He seemed to fancy himself as a bit of a macho man - karate, the SAS, etc. - and there are magazines (
Soldier of Fortune) and writers (e.g
The Poor Man's James Bond) who cater for this readership.
It is not exactly the same, but if he had read e.g.
The Day of the Jackal, he would have found in it a complete description of how to make explosive bullets that don't use any explosive. The Jackal needed only a hacksaw, fisherman's lead weights, a couple of thermometers, a soldering iron and a model-maker's
pin vise and metal files. In another book, the same author describes how to shoot quietly by reducing the powder in a bullet and making a silencer out of a 2-litre plastic Coke bottle. This is just stuff I recall coming across without even having looked for it. It is all out there for those who do.
The conviction didn't hang on that - there was much else that pointed to BG. The
appeal hung on it, but Nick Ross' blog article explains why the speck should not have been discounted - essentially, contamination requires a source and the possibilities were not in fact remote if properly considered.
The Serb thing is an interesting theory, but for me it stumbles on two things. One that if a rogue country's intelligence service had ordered a hit using a specially-adapted weapon, it would surely have leaked by now from somewhere at that end: the shooter, the getaway driver, the document forger, the armourer, the controller.
The other is that rogue states usually want everyone to know who did it. The idea is, you know what exactly happens to you if you are the next to cross them. We have multiple live examples, e.g. Litvinenko and Skripal, of Putin murdering his critics abroad as a deterrent. In each case the murderers took care to use a weapon that only a country could obtain - polonium, Novichok - to make it clear who did it. Inside Russia, those murdered always "fall" from high windows - it's the signature.