Tere are a stack of things to consider when specifying a spring or indeed designing one from scratch for a specific application. Material is obviously one choice, the type of spring may dictate the material profile but in a simple(ish) coil spring the profile is commonly circular although BSA owners will recognise square or perhaps rectangular sectioned springs. The latter more commonly in pairs to counteract 'Unbalanced forces' that can & do arise effecting both expansion & sompression characteristics.
The ends of the spring are design choices under the control of the designer, they may be open, closed, squared un-ground or ground. The external dimensions are also under the designers control. Pitch, external diameter at rest, number of active coils, total number of coils & length at rest diameter of material used or x sectional area of non circular material. Grinding is not always required as in some cases whera a small wire diameter is used or the ratio of wire diameter to outside diameter permits. But for air rifles the dimensions are such that grinding & polishing the ends has merit. Plain ended springs can bind in compression & so ground or polished ends are often preferable in many other applications too.
Expansion on compression needs to be allowed for.
A spring should ideally be capable of being compressed to its solid height & the desired load at the correct compressed length. The spec may be expressed asa percentage. Sometimes load at other lengths my also be specified too.

Generally air rifles use one spring, that's complex enough but two springs have been used & ideally they should in theory be wound in opposite directions. Not sure if Relum ones are or not but in theory it should be a benefit.

I mentioned expansion....this is something which I suppose could be regarded as 'instabillity' too. If a spring is long relative to its diameter then it will probably buckle as its compressed. In fact it's not really a probability. It will buckle. That's why guide rods/ tubes are used but the spring will in all likelihood push against the surfaces of these to some degree or another. Technically buckling happens when the springs deflection expressed as a percentage of its free length equals or exceeds a factor that was called 'the slenderness ratio'. Which was the free length divided by mean diameter.


Ok...

That's probably enough typing from me. Goodness knows how many spelling mistakes there are or what auto correct thought was a good idea but I fancy some lunch.