Originally posted by Blue Train
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First, have any of you experimenters noticed the same degree of shot-to-shot variation with automatics, which have no gap? And on the same subject, does barrel length of an automatic seem to affect variation in measured velocities, all other things being equal?
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One thing which can cause high extreme spreads in a semi-automatic is a weak recoil spring. If the recoil spring is weak, the slide may open a bit prematurely and allow more propellant gas than usual to escape out the breach rather than propel the bullet (a small amount of gas leakage from the breach is normal and unavoidable). Buffalo Bore actually has data specifically about this on their website regarding their 10mm ammunition. Tim Sundles states that installing a heavier recoil spring when shooting his 10mm ammo dropped the extreme spreads in one gun from 70fps to 35fps and from 100fps to 50fps in another.
https://www.buffalobore.com/index.ph...t_detail&p=114
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Second, does factory ammunition for .357 and 9mm Luger ever use the same bullets?
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While they may use bullets of the same design, they are not exactly the same as .38 Special and .357 Magnum normally use .357" jacketed bullets while .380 Auto, 9mm Parabellum, .38 Super, and .357 Sig normally use .355" jacketed bullets, in both calibers, cast bullets may be 1-2 thousands larger than jacketed. To my knowledge, however, Speer Gold Dots are the only commonly available JHP bullets which have significant design differences between one caliber and another. With most other bullets like Remington Golden Saber, Winchester Silvertip, Hornady XTP, and Federal Hydra-Shok, a .357 Magnum and 9mm driving the same bullet design of the same weight at the same velocity can be expected to perform very similarly to each other.