Let's put it this way. You SHOULDN'T have to specify the chamber, only the caliber. That's all it asks for on the form. Speak to BASC about it. My FEO told me specifically that under a .22 I could hold ANY .22rf calibre round, so .22 short, .22 long, .22LR and .22wmr. The .17 calibres are similar, .17rf rounds.
Some forces might try and limit it, don't let them!
There is still an arguement to say does it matter, if you can't make your mind up when you apply, and then buy one, you are stuffed, you will need a variation for the next anyway.
The fact is you are meant to justify by