Clint, if my trigger group is loose in the stock, what should I do?
When there is no clamping pressure (rattling trigger group), even a new trigger guard will not do much at all. When there is small, slight clamping pressure with a worn, flattened guard, a new guard will add a great deal to the clamping pressure. The linear difference here is very slight (just the measure the difference between a round & a flat trigger guard lug & you'll get the point). Unfortunately, the vast majority of used stocks in the world are well worn/sanded, and just a trigger guard will do little.
Thanks for your attention.
There are two dimensions involved in generating proper clamping pressure: the trigger guard lug diameter and the height of the stock from the trigger housing surface at the bottom to the receiver mounting surface at the top. If you have some clamping force, but not "enough" (this is somewhat subjective, of course), and the lugs are flattened, then a new trigger guard should do the trick. However, if there is little or no clamping force, the problem is likely that the stock is too "short" in the above referenced dimension. Remember that the purpose of the clamping force is to grip the receiver tightly in the stock.
If the stock is too "short," a consequence of years of clamping having crushed the wood fibers or the like, there are two remedies: a new stock, or glass bedding the trigger housing mounting surface. This latter was a common practice during the '60's rebuild program. Several M1s I have rehabbed for our local Police Department have such reclaimed stocks. You can see the glass plainly when the trigger group is removed.
Now, if your trigger guard is unlocking during shooting, that may be a consequence of wear of the trigger guard locking lug or deformity of the guard such that the locking force is insufficient. You do not mention if this is a stamped or a forged/milled guard. The guard should lock into place with a definite "snap" and require some force to disengage the locking lug from the trigger housing.