First let me welcome you to the forum. The Driving Spring for all three trainers is 9 1/2 inches long. You can cut down a new standard M1919A4 driving spring to fit. I would start at 10 1/2 inches and take it down from there. But TM 9-1205, page 122 calls for 9 1/2 inches. I have found that the M3 and M4 Trainer conversions works quite differently from .22 Long Rifle ammunition that was issued back in the 1940's and early 50's. There are several other folks that can help out here also. If there are any other questions please ask.
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