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Thread: Flashhole diameter question

  1. #1
    Boolit Master
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    Flashhole diameter question

    Not sure where to post this but could someone elaborate on what effect (if any) enlarging the size of the flashhole has on ignition of powder charge. It would seem that the diameter could be increased somewhat and still allow the anvil within the primer adequate support. Was thinking about the kicker of 4227 being used to ignite wc 860 and whether enlarging the flashhole might improve ignition of the 860.
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  2. #2
    Boolit Master

    nicholst55's Avatar
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    Enlarging the flash hole cause all manner of mayhem - if you overdo things. Larger flash holes allow more pressure against the primer when the cartridge is fired, which can lock up a revolver cylinder, or cause gas leakage around the primer resulting in a damaged breech face and firing pin.
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  3. #3
    Boolit Master
    georgerkahn's Avatar
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    I used to shoot primer-fired rubber bullets out of my .38 S&W Special revolver, perhaps twenty-five years ago. If you seated a rubber bullet using a regular or even a magnum primer in a "virgin" case, the bullet had maybe a six foot range before it (gravity works) fell to ground. On the other hand, if you drilled the stock 0.080" hole to be much larger (for safety, I will not offer the enlarged diameter), the same rubber bullet, in same case, with same brand/type primer, would then be able to perforate a piece of "shirt cardboard" (old timers remember there used to be a piece of cardboard packed with new store-bought shirts ) at 15 feet with amazing accuracy!
    This should help answer your questions... The simple answer to your query is either -- take your pick -- the one-word, "NO!", or, three-word, "DON'T DO IT!!!" Along with the flash-hole enlargement directions from Speer with their rubber bullets, too, were instructions to mark cases which you enlarged the flash hole so they might not inadvertently loaded/mixed up with factory cases -- to avoid catastrophic injury to shooter and/or firearm.
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  4. #4
    Boolit Master

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    Enlarging flash holes has some detrimental effects also. Increased pressures on primer, when fired in full power loads. possible leakage of said pressures. Primers are anvils are supported on the out side legs of the anvil so support isn't really an issue. Most Ream flash holes to a consistant dia only enlarging the smallest to the biggest size in the batch. Find the biggest and the number drill that fits into it with light resistance and ream the others to it. another aid to ignition is to uniform flash holes inside the case applying a light chamfer and removing any burrs present. This greatly increases the ignition consistency.

    I made up a couple hundred 38 spl cases for wax bullet loads for use in snubby revolvers. Like rubber and plastic bullets flash holes need to be opened up to allow better flow thru. When I did this I ground a drill to a pilot for the flash hole and large primer pocket and cut the cases to large pistol pockets and the oversized flash hole. This way the cases could never be mixed in with 38 spl being loaded. On the 44spl and 45 cases I sleeved them down to small pistol primers again to keep from being mixed in.

    If you decide to open up flash holes cases need to be ided so they are known at a glance to be modified.

  5. #5
    Boolit Master Outpost75's Avatar
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    I have reloaded .38 S&W, .38 Special and .45 ACP cases which came from the factory with. 120" diameter flash holes, loading standard pressure charges for the caliber, with standard lead bullets for the caliber, 150-grain in .38 S&W with 2.5 grains of Bullseye, 158-grain lead SWC in. 38 Special with 3.5 grains of Bullseye, and 230 FMJ in .45 ACP with 5 grains of Bullseye. Have shot thousands of rounds in a variety of guns with no issues. Winchester wouldn't sell bulk commercial quantities of these as factory primed brass if it was dangerous.
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  6. #6
    Boolit Master
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    I conducted the following experiment to determine the effect of enlarging the flash hole on /minimum charges/ of IMR4895 in an M1 Garand - /after/ reading what Sharpe had to say about this specific subject in his seminal work, "Complete Guide to Handloading". Highly recommend reading if haven't already read - my notes indicate that Sharpe tested flash holes enlarged to ~0.101". He did obtain substantial increases in chamber pressure, but minimal to no increases in muzzle velocity.

    -----

    I found the minimum charge of IMR4895 that would cycle my Garand (40 grs gave good accuracy, 38 grains was minimum for reliable function).

    36 Grains did not give reliable function from the standard 0.080" flash hole.

    Quickload indicated that the pressure at my gas port from the 38gr load was about 6200 psi, compared to 8000-9000 from a standard M2 ball load.
    My hope was to improve powder ignition & initial burn in a below-minimum load, obtaining a bump in chamber pressure that would also translate to increased pressure at the Garand gas port - and subsequent improvement in function.

    I then enlarged flash holes with a series of drill bits, closely looking at the condition of the fired primers in all cases to determine if any issues were imminent.

    I first used 7/64", then 1/8" (~0.125).
    While relatively-common reduced load techniques utilize up to a 9/64" drill bit (~0.140"), these minimum loads I was testing were technically /not/ reduced loads, so I didn't want to open flash holes that far.

    Even with flash holes opened to 1/8", I was not able to obtain reliable function at 36 grs of IMR4895 - indicating that if any significantly-increased pressures were actually occurring (admittedly unlikely from looking at fired primer appearances), those pressures were not resulting in a better burn of the below-minimum powder charges.

    I would consider this to be a very controlled test; with close examination of Quickload calculations & the predicted pressure curves, along with the chrono data. I was also confident on the ability of the M1 Garand action to provide an increased safety margin for my tests.

    Hope this helps,
    Last edited by Kestrel4k; 07-23-2018 at 08:29 PM.

  7. #7
    Boolit Master


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    Fyi. Speer does this on factory 357 sig brass. SMALLER than normal flash holes.

    Makes reloading them a BUGGER!! As it pulles non headed de capping pins out after a few rounds. Grrr. Usually leaving them stuck in a case.

    Progressive its a true headache.

    Now i simply universal de prime tham in a mighty armory tapered Decaping pin. It nicely swadges the flash hole up to standard size.

    I load some very warm loads in the lil sig with zero problems.

    CW

    CW
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  8. #8
    Boolit Master

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    I was loading some 45ACP the other day and found 3 brass with dramatically large flash holes.
    I tossed them in the junk brass tub. That is one advantage of single stage reloading.
    I probably would not have caught them on a progressive....dale

  9. #9
    Boolit Master
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    Quote Originally Posted by dale2242 View Post
    I was loading some 45ACP the other day and found 3 brass with dramatically large flash holes.
    I tossed them in the junk brass tub. That is one advantage of single stage reloading.
    I probably would not have caught them on a progressive....dale
    I think S&B did this in the tirst runs with their new leadfree primers (of dubious quality). They also staked them into place of the same reason (big holes and big backpressure)

  10. #10
    Boolit Master


    Soundguy's Avatar
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    i'd also avoid duplexing a load... ( of smokeless anyway ) your mileage may vary..

  11. #11
    Boolit Master

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    I was going to say that opening works both ways. Keeping it small throttles the pressure coming back to the primer cup. Some guns with poorly fitted firing pin holes/ firing pin diameter can rupture. There are also some good consistency issues. Look at the match .308 casings with small primers and small flash holes. Stronger case too.

  12. #12
    Boolit Master Outpost75's Avatar
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    In .45 ACP I ran velocity checks with 5 grains of Bullseye, 230-grain FMJ, full charge hardball equivalent, Winchester LP primers with large .120" flash holes as loaded by the factory for Win-Clean range ammo and standard 0.080" flash hole from Winchester Walmart white box ammo, NO DIFFERENCE in avg. velocity. Large flash hole had smaller Sd
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  13. #13
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    Larry Gibson's Avatar
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    Often I recommend drilling out flash holes in rimless cases for use with very light loads to avoid case headspace set back. I recommend a 30 to # 28 drill for this with cases using LR primers. With small charges of fast burning powders under cast bullets this works well.

    I have measured the psi and velocity of .308W cases with the flash holes so drilled with loads using 4895 and 311291 cast bullets up through 2200 fps alongside identical cases with identical loads without the flash holes drilled. No increase in pressure or velocity was measured however the ES and SDs were most often lower with the drilled flash holes.
    Larry Gibson

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Abbreviations used in Reloading

BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
C Compressed Charge PR Primer SPCL Soft Point "Core-Lokt"
HP Hollow Point PSPCL Pointed Soft Point "Core Lokt" C.O.L. Cartridge Overall Length
PSP Pointed Soft Point Spz Spitzer Point SBT Spitzer Boat Tail
LRN Lead Round Nose LWC Lead Wad Cutter LSWC Lead Semi Wad Cutter
GC Gas Check