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2: Pressure will be higher. This will cause you to have to use reduced handloads to get safe pressures. This doesnt meen that you cant run max velocity potential, just that it will take less powder to get there.
That said, you may not be able to use factory ammo in a 'match' chamber with a short throat even though they will fit as the pressures can be too high. If you want to run factory ammo, you will need a full SAMMI spec chamber AND a decent amount of freebore. If you only want to hand load, the tighter chamber and short throat may be the best option. However, I would also have you follow the other posts advice and double check with your smith about just exactly what he meens. Unless you are building a bech gun, you do not need a tight neck chamber. When it comes to accuracy, tight necks are HIGHLY over-rated in the first place.