Gary,
I agree with everyone here, and Sinclair's explanation makes sense but is arguable. As you know necks creep forward over a number of firings and if not checked will eventually crimp into the bullet. You don't want this for safety reasons. Some necks will be longer than others and as they start to crimp into the bullet the pressure builds and velocities are all over the place, plus the obvious safety hazard.
How long your neck is depends on the reamer used on your chamber - every chamber is different. One way to figure it out is to take a cast of your chamber (I have never done this). Another way is to use the Sinclair inserts in Jasper's post, or make your own if you have access to a lathe.
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Sheridan rifle gauges are apparently reamed with a normal chamber reamer so this picture shows typically how much clearance you might have before the necks hits the forcing cone of the barrel.
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As for the Sinclair advice, it is fair but I would not use it as an excuse to let your cases creep forward as there's arguably no advantage in letting this happen - even on a semi-auto. With Grendel, there is typically plenty of grip on 6.5 bullets so being even more generous with neck length is of no advantage. Sinclair's idea of keeping cases clean by letting the necks grow up to the forcing cone is arguable. Risk/reward is just worth it. Regardless, carbonised necks or stained cases don't affect accuracy anyway. Cleaning cases is more cosmetic than useful. Unless you trim your cases to a stupid shortness they are always going to obdurate and seal barrel gasses.
Best-practise is to trim cases to SAAMI and then you don't have to worry about it. In Grendel they don't creep forward too much anyway. Whatever case length you settle on it pays to keep all cases as consistent to that length as possible for accuracy.
I agree with everyone here, and Sinclair's explanation makes sense but is arguable. As you know necks creep forward over a number of firings and if not checked will eventually crimp into the bullet. You don't want this for safety reasons. Some necks will be longer than others and as they start to crimp into the bullet the pressure builds and velocities are all over the place, plus the obvious safety hazard.
How long your neck is depends on the reamer used on your chamber - every chamber is different. One way to figure it out is to take a cast of your chamber (I have never done this). Another way is to use the Sinclair inserts in Jasper's post, or make your own if you have access to a lathe.
[ATTACH=CONFIG]18482[/ATTACH]
Sheridan rifle gauges are apparently reamed with a normal chamber reamer so this picture shows typically how much clearance you might have before the necks hits the forcing cone of the barrel.
[ATTACH=CONFIG]18483[/ATTACH]
As for the Sinclair advice, it is fair but I would not use it as an excuse to let your cases creep forward as there's arguably no advantage in letting this happen - even on a semi-auto. With Grendel, there is typically plenty of grip on 6.5 bullets so being even more generous with neck length is of no advantage. Sinclair's idea of keeping cases clean by letting the necks grow up to the forcing cone is arguable. Risk/reward is just worth it. Regardless, carbonised necks or stained cases don't affect accuracy anyway. Cleaning cases is more cosmetic than useful. Unless you trim your cases to a stupid shortness they are always going to obdurate and seal barrel gasses.
Best-practise is to trim cases to SAAMI and then you don't have to worry about it. In Grendel they don't creep forward too much anyway. Whatever case length you settle on it pays to keep all cases as consistent to that length as possible for accuracy.