The o-ring used was long ago identified by Busta, and can be found on this page:

http://www.mcmaster.com/#catalog/118/3490/=iuirh7

It is the one with the 4.5mm ID, # 9262K611



Using these o-ring results in zero blow back. The reason i use these o-ring, is because eventually all of the breech plugs in my rifles suffer flame cutting of the primer seat. This allows for more and more and more blow back. What is done then, is the flame cut is removed by drilling the primer seat deeper, by using a drill i have ground much flatter than standard. This makes the bottom of the primer pocket smooth again, but now the primer isn't long enough to make a seal. The o-ring replaces the drilled away steel with a flexible material that makes an excellent seal. Generally, the primer pocket is made about 0.020" deeper. The o-ring is about 0.040" thick. The o-ring being a tad thicker than the amount of material removed compresses to make a 'perfect' seal. Using these o-ring results in zero blow back.

My guess is you won't be able to use these o-ring in your Omega without somehow increasing the headspace. Either by unscrewing and shimming the firing pin bushing, or by drilling the primer pocket deeper. However, the CVA rifles i own can utilize these o-ring without doing any work.

A long range session for me is about 30 shots. The o-ring easily survives that many shots. A short range session might be 10 shots, after a short session the o-ring can be used again more than once. These o-ring seem to 'evaporate' as shots are taken. When they become 0.035" thick or less, i quit using them. I don't actually count how many shots they are good for, but i would guess about 50.

Myself, i like using these o-ring. Because they make a perfect seal, the primer seat will never again flame cut. The breech plug will last and last with no way for it to be damaged. The vent liner can be replaced when it wears after perhaps 200 shots, and the o-ring can be replaced every 30 shots or so. Cost of the o-ring is less than a nickel.