I've used the Wolfe M1A complete spring packs on many rifles now and they are excellent. Tubb springs is another one a few cgn'ers seem to like but i have little experience with them and have not run them in my rifles. The standard M1A springs from brownells are springfield and they seem to be of fairly good quality as well. While I generally recommend swapping the springs out in general to thier U.S. made equivalents, in this case, it may or may not solve the problem.
if the extractor spring (chinese variety) mod does not correct ejection angle enough to stop Arms 18/oprod jamming..... you can rule out other possible causes. with your oprod fully forward in the closed bolt position, does it have much up and down play when you wiggle the oprod handle? (fix.... new or tighter fitting used oprod)
Is your oprod guide slightly or excessively loose on the barrel?
These two things can also contribute to the ARMS 18/oprod jamming issue as they can allow the oprod to rise at it comes forward or allow it to have an eratic path as it comes forward, occasionally jamming a case between oprod and mount. If neither of these symptoms are present, perhaps a spring change and inspect extractor for wear, possibly order an M1A one while ordering springs (get a new extractor detent as well)
I'll add as an after thought...... try some different ammo..... it's kinda grasping straws but with this issue it's all about ejection angle and speed of ejection. Most of my rifles if not all shoot .308 commercial spec loads and ejection is violently out and away to the side...... although one of my rifles simply spits them into a nice pile on the bench, right beside the rifle hehehe.
you want to acheive an ejection angle that is under 90 degrees from the receiver..... if the rifle has a tendency to eject slightly forward, or past 90 degrees...... the arms 18 jamming issue is inevitable. this is why the extractor spring is trimmed, to change the ejection path.
