When loading for the AR-15 or any Gas Gun, we have found that H335 is hard to beat, Winchester 748 is also a good powder in the Gas Guns. We get less carbon build up on the gas tube and bolt, as we do with other powders. As for a load that is very accurate in a Gas Gun try 25.5 gr. H335 - 55gr. Hornady V-MAX - Winchester Small Rifle Primer - Government Case - OAL 2.210 or try 26.0gr. H335 - 50gr. Hornady V-MAX - Winchester Small Rifle Primer - Government Case - OAL 2.200.
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The government cases seem to hold up better than commercial cases, but when using government cases back the powder charge down 10% below that you see listed in loading manuals for commercial cases. This is due to less case volume in the government case as in the commercial case.
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When loading for any Gas Gun always Full Length Size using a Small Base Sizing Die & always use a Head Space Gauge to setup your Loading Dies before loading or you WILL get Dangerous Slamfires.
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Your 1/9 Twist will shoot all bullet weights accurately. We do not recommend anything below 50gr. for any 1/7 twist. Remember you do not need a heavy bullet for a fast twist! You do need the fast twist to stabilize the (longer) heavy bullets.</P>