Some .22 rim thickness measuring

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Re: Some .22 rim thickness measuring

Post by Wyliecoyote » 13 May 2021, 5:51 pm

HV rimfire ammo does not enter into the conversation when it concerns accuracy. It is generally launched just forward of the sonic shock wave and is then subject to the transonic area shortly after where its less than perfect base form is subject to some element of instabilty. You could have 1fps ES and shoot a group of any size. Hence the reason, amongst a few others like more wind deflection is why no such ammunition is used in competition.
I grant that some ammo can give great figures and shoot poorly no matter what you do. Primarily the reason for that is bullet slump or base obturation, or lack of it if the alloy is too hard, caused at peak pressure. Nothing anyone can do about that except move on or do what Anschutz and a few others do and have choked bores. But even that won't save some brands.

Then there is the case of barrel time, a position where again the numbers are good, but bullet exit time is not. A question often asked of centerfire shooters testing over a chrono and getting single digit numbers but horrid groups. The simple explanation is ignition occurs but the time it takes to occur varies. The peak pressure builds the same as the other giving near identical velocity but the pill exits at a different barrel moment thus destroying any chance of a group. Or to be put very simply, from the time the firing pin snaps to the exact time the bullet leaves the muzzle varies. Ever so slightly, but enough milliseconds to catch the muzzle at a different moment. How that delay is caused in rimfires is fairly well established.
Federal 900A, 900B and both Ultramatch versions were highly regarded and sought after ammunition because of the dimple base that evenly distributed the primer mix to the outer circumference where it was needed to be pinched so a "thin" spot of primer mix wasn't caught. Easy fix to a known issue that bewilders many as to why others don't do it.

The other alternative was multiple firing pins like Time Precision did, or more recently Stiller Copperhead 2500 series actions use. The theory is start the fire at both ends of the house and it burns four times as fast producing better ESs with even average ammo.

The fact is that rimfires can be a devil of frustration to even the most experienced. The other fact is that to get the right primer distribution and bullet alloy hardness to obturate consistently costs a lot of money these days. No rimfire ammo in my view is worth anywhere near the asking price. Hence the reason i stopped shooting rimfire BR the day that Federal ceased production of its Match line. The current stuff they offer is an insult of both price and performance.
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Re: Some .22 rim thickness measuring

Post by bladeracer » 13 May 2021, 6:35 pm

While some, perhaps most, bullets suffer some degree of instability during transition, it is not a given. Some supersonic bullets drop back to subsonic with no significant stability issues.


Wyliecoyote wrote:HV rimfire ammo does not enter into the conversation when it concerns accuracy. It is generally launched just forward of the sonic shock wave and is then subject to the transonic area shortly after where its less than perfect base form is subject to some element of instabilty. You could have 1fps ES and shoot a group of any size. Hence the reason, amongst a few others like more wind deflection is why no such ammunition is used in competition.
I grant that some ammo can give great figures and shoot poorly no matter what you do. Primarily the reason for that is bullet slump or base obturation, or lack of it if the alloy is too hard, caused at peak pressure. Nothing anyone can do about that except move on or do what Anschutz and a few others do and have choked bores. But even that won't save some brands.

Then there is the case of barrel time, a position where again the numbers are good, but bullet exit time is not. A question often asked of centerfire shooters testing over a chrono and getting single digit numbers but horrid groups. The simple explanation is ignition occurs but the time it takes to occur varies. The peak pressure builds the same as the other giving near identical velocity but the pill exits at a different barrel moment thus destroying any chance of a group. Or to be put very simply, from the time the firing pin snaps to the exact time the bullet leaves the muzzle varies. Ever so slightly, but enough milliseconds to catch the muzzle at a different moment. How that delay is caused in rimfires is fairly well established.
Federal 900A, 900B and both Ultramatch versions were highly regarded and sought after ammunition because of the dimple base that evenly distributed the primer mix to the outer circumference where it was needed to be pinched so a "thin" spot of primer mix wasn't caught. Easy fix to a known issue that bewilders many as to why others don't do it.

The other alternative was multiple firing pins like Time Precision did, or more recently Stiller Copperhead 2500 series actions use. The theory is start the fire at both ends of the house and it burns four times as fast producing better ESs with even average ammo.

The fact is that rimfires can be a devil of frustration to even the most experienced. The other fact is that to get the right primer distribution and bullet alloy hardness to obturate consistently costs a lot of money these days. No rimfire ammo in my view is worth anywhere near the asking price. Hence the reason i stopped shooting rimfire BR the day that Federal ceased production of its Match line. The current stuff they offer is an insult of both price and performance.
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