How critical is primer flash hole uniformity

5mmgunguy

How critical is primer flash hole uniformity? To velocity uniformity? Accuracy?

p3skyking

In small arms ammo, not that critical. You have to have the flame ignite the powder, but in reality, that's about it.

beantownshootah

If there is no flash hole, accuracy will suffer. [;)]

"Critical" is sort of subjective, but I think this can make a difference.

Obviously, maximum consistency is the key to accuracy.

Relative to things like powder charge, bullet quality, case length and seating consistency, etc, I think primer flash hole consistency is USUALLY less important, EDIT. . .assuming you're using good brass where the flash holes are already consistent from the factory. It will be more of an issue if you're mixing lots of brass where flash holes may be different diameters Match type shooters who are concerned about achieving maximum possible accuracy will generally uniform primer holes. I don't think this is pure superstition on their part.

Relative contribution to accuracy of primer hole consistency is going to depend on the nature of the load in question. I think it matters more in loads with large volume cases and slow burning powders where its harder to get consistent ignition of all the powder. Also, for most loads its probably more important that the flash holes just all be the same diameter then that they be any given size.

I think the biggest reason to uniform the flash holes is just to get rid of burrs. Obviously, gross defects in flash hole (eg way over or undersized, or ones partly blocked by burrs, etc) are going to be the worst offenders in terms of affecting accuracy. Also, if you're using brass from different makers, the factory flash holes can be different diameters, and uniforming them certainly can't hurt.

RCrosby

Assuming good brass, same lot, etc.; if you're shooting in the "5's" and up, I wouldn't worry about it. If you're determined to get down into the "1's and 2's", and have the rifle that can do it, then I'd play every card in the deck, including uniforming flash holes.
Nothing in stone, just my own opinion. We all have some.[:D][:D]

mbsams

Just another variable easily delt with. You only have to do it once. So do it!

USN_Airdale

aaaaaaah !! one of my favorite subjects, primer flash holes, over the past 5 years i have experimented with flash holes which are .080" in rifle primer sizes, i have opened them up to .085 velocity was a bit higher, in my .499LWR, i have made a 45 degree angle inside the case this too increased the velocity and consistency in velocity, chronograph read outs in a 10 shot string gave me 4 "dup vel." (duplicate) readings and the spread was 23 FPS, amazing.., huh ? then i drilled out the flash hole tapped threads in them at 6-48 TPI took brass tubing, commonly used in model airplane fuel tank tubing, cut 6-48 TPI on one end and screwed in the tube tightly, used my tried & true load and chrono'ed them, WoW ! what a difference in velocity and accuracy, vel. increased on an average of 150 FPS, accuracy at 100 yds. gave me 10 shot, 2" groups, the reason for this is that the powder is ignited from the front and burns toward the rear of the case giving a more uniform burning and nearly 100% powder burn, inspection of the rifles bore after each shot proved that.

military large bore ammo such as the 40mm and 105mm cases have a tube in them that reaches the front of the powder charge, that is what got me to thinking about getting a good clean burn of the powder charge. as for the .223 Rem./5.56mm NATO, all i could do with them was ream the inside of the F.H. to a 45 deg. angle, swaging the 45 deg. angle is a no,no, as it pushes the primer pocket web back enough to keep the primer from seating to full depth, reaming is the best way, velocity & accuracy was substantially better. i used a Dremel tool bit in a small pin vise to ream the 45 deg. angle.

i am convinced that for straight wall large bore cases a frontal ignition is far superior to rear ignition, as for general primer flash holes, they are punched out leaving some irregular burrs, so deburring them will also give slightly better ignition.

JustC

if you aren't shooting benchrest, with a benchrest rifle, it will show very little increase in accuracy.

I do it for factory rifles simply because I like to sit at the load bench[;)]

charliemeyer007

I use the Lyman flash hole tool on everything I can. Not sure it helps a lot but you only need to do it once and it's easy. I remember reading about the 333 OKH. Those guys were fooling around with lighting the fire at the base of the bullet pre WWII. They kepted it on the down low for fear the huns would employ the idea. I think the O was O'Neal the K was Kieth and don't recall the H. Anyway decapping was real chore but did produce higher velocity and the muzzle end got hot first.

riley p

I don't know USN Airdale but I can back his observation. Some years back I and a couple fellow hi-power shooters set to find out what was "really important" and what was not in reloading ammunition.
First was bullet quality, and surprisingly second was flash hole uniformity!! Third was primer "quality". Brass uniformity, powder weight and "all the other variables" came in a distant fourth.
I use a flash hole uniformer on all new brass (only have to do it once) and consider it time very well spent.

5mmgunguy

Thanks guys!

nononsense

5mmgunguy,

I see that I'm late to the party...

The uniformity of the flash hole is critical to consistency no matter what others opine. Who will notice it, under what circumstances, is the question. This will determine the value of the process. You would have to eliminate a bunch of other variables before hand to be able to qualify the difference under many average circumstances. I suggest a top quality chronograph suited to process, not the junk being touted these days by nearly every manufacturer, coming in from China. The type of brass or the manufacturer will also determine the need since it is not usually necessary to process Norma or Lapua brass. The generic Winchester/Remington brass always requires the full spread of accurizing due a decreasing lack of quality from the factories.

This is a quote and link to the blog from Sinclair International where they covered this topic:

"Ron Dague: "I feel that the most important step(s) in reloading for accuracy are in the initial case prep. Uniforming the primer pocket to the same depth to ensure consistency in primer seating is a crucial step. Additionally de-burring the flash holes, each in the same way to clean up and chamfer the inside is important. It ensures that the ignition from the primer is uniform and flows out in the same consistent pattern. Doing so will create uniform powder ignition and tighten up your velocity Extreme Spread."

{Elsewhere}.com: With some brands of brass, primer pocket uniforming and flash-hole deburring is useful. However, with the best Lapua, Norma, and RWS brass it may be unnecessary, or worse, counter-productive. So long as your Lapua brass flash-holes are not obstructed or smaller than spec, it may be best to leave them alone. This is particularly true with the small flash holes in 220 Russian, 6BR, and 6.5x47 cases. MOST of the flash-hole reaming tools on the market have cutting bits that vary in size because of manufacturing tolerances. We've found tools with an advertised diameter of .0625#8243; (1/16#8243;) that actually cut an 0.068#8243; hole. In addition, we are wary of flash-hole deburring tools that cut an aggressive inside chamfer on the flash-holes. The reason is that it is very difficult to control the amount of chamfer precisely, even with tools that have a depth stop."

http://blog.sinclairintl.com/2010/07/

Not quite 1/2 way down the page.

Best.