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Discussion Starter · #1 ·
Now i understand that it makes sense that after i cleaned out my gas plug, and piston with the sadlak cleaning drills that all that carbon residue i removed will mean more volume inside my gas cylinder(man those tools work great). This should in turn lower my velocity just a hair, and it did.

I chronographed CBC brass, WLR primer, 168gr SMK a few days ago with 40.5gr H4895 at an average of 2622 FPS with the dirty cylinder (about 300 rounds through it prior without cleaning). After that shooting i cleaned the cylinder and my rounds today shot with 40.7gr H4895 averaged 2602FPS.

These results do make sense to my limited knowledge of the platform since more volume should equal less pressure before the bullet exits, I am just wondering if anyone else has seen similar results? I am using a prochrono digital and just recently started chronographing my reloads so my averages may not be as accurate as those of you with those dang nice oehlers.
 

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Was the ambient air temperature the same or close both days? it can make a difference, Atmospheric pressure can to. Was the powder from the same lot? there are a lot of things that change muzzle velocity from day to day. I have a weather panel that has small three gauges [temp, humidly and A. pressure] that I take with me when I do chronograph work and log it in a note book so I can compare it to other tests. You could probably do it with a smart phone now.

Casey
 

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I am just about 100% sure that the condition of the gas screw/system would have (virtually) NOTHING to do with this (minor) variation. Heck, DISABLE the gas system if you like and THEN compare if you want to..
 

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Discussion Starter · #4 · (Edited)
the temperature difference was at most 5 degrees farenhieght at the same shooting spot (same elevation), however the pressure may have been different due to a rain cloud above me. The powder was from the same lot, heck even the same can for these rounds. I am new to having a chronograph and did not think pressure would have much effect on my bullet besides its ballistics table and drop values at a distance, but it sounds like i was assuming when i shouldn't have.

If you guys have a suggestion for a decent device to measure temp, hum, and atm pressure i am all ears, i am one of the few people without a smart phone these days it seems despite being an electrical engineer.
 

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20 fps is insignificant. Chronograph may be the reason itself. Sunny day cloudy day. Mine works much better with clouds. .02 difference in the powder in nothing. Bore can make a difference too. Once it starts fouling a bit.
 

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The velocity difference is within the extreme spread you would normally encounter with most ammo. With the range you would normally shoot the 168 SMKs the velocity difference you noted will not make a difference.

If the load shoots, load more and burn the barrel.
 

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Discussion Starter · #7 ·
Thanks for the response, and i think i found the reason for no velocity increase with increase in powder. The other day i took my time and set the chronograph on a tripod level with my shots, today i was sighting in a few .223 and took the chrono out out of curiosity to check some new loads. I set the chrono on the ground and shot over it in a hurry to beat the rain cloud looming overhead, but the chrono may have been slightly off-level with the shots making them read slightly slow than they should have.

Either way i think i have found a decent average velocity in the 2550-2600 fps range, now i am just waiting on my sadlak mount so i can really see what the gun can do (my eyes are too poor to hit anything besides steel plates at 200-300yds)
 

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I notice no "number of rounds fired per string" often noted as being "N=" for any of the data.
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If you are shooting less than AT LEAST 13 rounds and preferably 20 rounds and more preferably 30 rounds per lot number the chance that you are going to get the true mean from something like a 5 round string is not very good.
Myself, if I can get a 13 round string of data with barrel cooling, etc, between shots I usually call that good for almost all purposes.
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I myself tend to shoot 5 shot strings to test lots and then if I'm reloading a big batch like a 500 rounder I will load 530 rounds and spend the time necessary for shooting 30 rounds across the chronograph to get a really decent reading for longer ranges like 600 yards.
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Collect the temperature, etc, data as others have mentioned. Over time trends will possibly appear in your data which can be put in your range card for that ammunition.
 

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Given the position of the barrel gas port (near the muzzle), I would think a fired bullet would exit the barrel before any gas plug / cylinder volume differences caused by fouling could have an impact on that bullet's velocity.
 
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