I think I will re-trim the 100 brass that is trimmed to 1.755" so I can match the brass that is already 1.75", so they will all be the same, I agree that consistancy is important for accuracy. I just didn't think I could expect much more than 1" groups at 100 yards out of my Stag M4 with a standard 20 " 1-9 twist barrel. If
I had a heavy stainless match barrel, I could expect to shoot .5 MOA or better. That's why I didn't think the 5 thousands difference in case length would change point of impact much at 100 yards, but I might as well do what I can to get the most out of the rifle that I have.
Your existing 20" 1-9" barrel and rifle can likely be "improved" to shoot .5 moa or better. I have built a lot of target and long-range varmint A/R's, but have also made some 20" 1-8 and 1-9 twist models. Here are some of the more noteworthy things for accuracy that you can do to your A/R:
1) Replace the trigger if it is over 4 or 5 pounds with a 2 1/2-3 lb. trigger. I'd not try to adjust to 2 pounds or under as you risk multi-shot bursts. This lighter trigger alone can see instant improvement to all loads.
2) Focus on a bullet in the 50-52 grain weight range. Me - I'd start with something like a 52-grain target bullet. Do not choose lead-tipped bullets. Use either hollow point or tipped bullets, with tipped being the best in an A/R. While that 1-9" twist will stabilize heavier bullets - start with the 52.
3) I'd be looking at trying these powders in this order (unless you have some of these already): Accurate LT32, Reloader 7, IMR8208XBR, TAC. There are LOTS of great .223 powders - but this is just where I start after having seen what worked best, most often, in a number of A/R's shooting the 50-grain range of bullets. There is no one "wonder powder", but I'd start with LT32.
4) Eliminate "slop" between the upper and lower receivers. There are devices one can buy to prevent this. Even the little $5.00 plastic "bumper" helps. Rattles and vibrations on the rifle are not good.
6) Replace the mainspring with a Wolff reduced power mainspring when shooting the 52-and-under weight bullets. You will likely get better results than using the stock spring which is designed for shooting any heavy bullet someone might put in the rifle.
7) Make sure your brass shoulders are only being bumped NO MORE than .003"-.004". This is HUGE!
8) Check your feed lips and bevel at the entrance of your chamber. If they have sharp edges on them, use a Dremel tool or such to abrade and polish the rough edges till they are smooth where the bullet can better glide over them without deforming or slicing the bullet tips.
9) Grab your fore-end with one hand and the butt-stock with the other. See if there is any rotational slop or play in the vicinity of where barrel meets receiver. If there is, remove handguard and barrel, apply RED Lock Tite liberally to areas where there are spaces lacking contact, then reassemble. You will need a heat gun or torch to remove the handguard and barrel in the future if you do this. But it will greatly reduce or eliminate the accuracy-robbing chatter and vibration cause by this loose fit. On a few rifles, I actually used JB Weld to permanently fuse the upper receiver and barrel/barrel nut together for the life of the combined unit if the upper was notably loose. If you do that and your hand guard does not have a separate barrel nut, beware that your hand guard also becomes consumable, along with the upper receiver and the barrel. I could take it to a machinist to shave, in order to time the action to being tight when aligning with the next gas tube hole - but a "stripped" upper is pretty reasonable in cost and will be more solid and vibration free in this manner.
10) If you shoot off a bench, make or buy a fore-end sled to attach to the fore-end of the rifle that will allow the rifle to slide freely in the rest. I shoot my AR's off a toggle rest when varmint shooting. Greatly reducing rotational torque when a shot is fired is meaningful - just as it is in bench rest shooting. If I'm shooting off the bipod, I use a leather squeeze bag for the rear. Bench rest shooters don't put bipods on their rifles, but folks think nothing of doing so with an A/R shot off the bench. For a lighter rifle such as yours - maybe nice to be able to do both.
Regarding your trim length being .005" off with other cases - I'd just trim them all the same as you have started. Frankly, I've shot mixed lots that were .010"-.015" off from one another without seeing the type of accuracy change some might think would occur. Just thought I'd throw in the above info in case you had thoughts that an existing 20" standard M4-style A/R can't be made much better. There is a lot more that can be done - but these things should put you at or under 1/2" MOA with good loading practices. You don't need a terrific (or bull) barrel to get 1/2" in an A/R if your overall rifle and load is dialed in. Of course, powerful enough optics to shoot tiny groups is necessary. I'd not want anything less than 10x to 12x on the high end. On my target and varmint rifles, I have a minimum of 30x top-end.
Outside of the scope and trigger, all else noted is relatively inexpensive to do. If there is anything here that helps if you opt to tune that rifle, that is great.
Good Luck
