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Data on Cones, Bores, and Chokes

Discussion in 'Uncategorized Threads' started by BDodd, Apr 17, 2007.

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  1. BDodd

    BDodd TS Member

    Jan 29, 1998
    I ran across this article in "Sporting Clays Magazine" and it will come here in 3 separate postings to keep the size down to a readable amount for each. And, yes, I'm just about ready to begin typing again so be prepared. The first portion, below, is intended to advise folks about lengthening forcing cones. Enjoy and expect two more threads in coming days....Bob Dodd

    The following information comes from an article in the May 2007 Sporting Clays Magazine and was written by Tom Roster, a respected scribe for things of the target shotgun world. He is from Klamath Falls, OR and has written reloading manuals on buffered lead and bismuth shot shells loads, new HEVI-SHOT reloading, instructional videos, and shotgun barrel alterations. I can't speak for his accuracy but quote his points on these subjects.

    Forcing Cone Length
    "...I started with two different 12-ga shotgun barrels, one possessing a standard bore of .730", the other over-boared to .750". Then, as the cones in each barrel were lengthened, in 1" increments, from the existing standard of 1/2" to something much longer, I measured the effect on velocity and patterns on one 3-dram (1200-fps) 1 1/8-oz, 12-ga. lead shot target load and one 1365-fps, 1 1/8-oz, 12-ga. steel shot load.

    "I found that the patterns of the lead shot load improved slightly (got denser at 30 to 50 yard distances) as the forcing cone was lengthened in both barrels by 2" (2 1/2" total lenth). However, as the forcing cone in each barrel was lengthened to 3" and then to 4", I could measure no further improvement in the pattern performance in either barrel no matter what the finished forcing cone length.

    "In terms of velocity, I could measure no change for either the lead or steel load in any of the finished forcing cone lengths out to 4" (4 1/2" total length) in the .730" barrel. With the .750" over-bored barrel, however, no significant velocity loss occurred to either load until the forcing cone's legth exceeded 3" (3 1/2" total). Then, a velocity loss of about 30-fps occurred for the lead load with about a 75-fps velocity loss for the steel load. I attribute this to powder gases escaping around the obturating skirt of the lead shot load's wad - and especially the steel shot load's wad - as the internal diameter of the forcing cone/bore increased in size.

    "The lesson? Forcing cone lengthening never helps and sometimes hurts velocity levels. With soft shot types such as lead, forcing cone lengthening can improve patterns somewhat. The benefit, however, occurs more with large shot sizes than with small shot sizes. Last, from my testing, I cannot see much, if anything, to be gained from having a forcing cone lengthened past a finished length of 2 1/2", 3" max."
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