Lapua brass burrs on the flash hole.

[That bald headed Geordie]

After reading on 6BR website the many comments regarding the benefits of the removal of internal burrs on the flash hole on Lapua brass. Some of the comments say the burrs should be removed as it will affect the powder burn, and others say do not bother just load and shoot. Being a benchrest / f class shooter in the main, I try to get the best out of my loads by doing all the usual anally retentive stuff (weighing/batch sorting brass and bullets etc). When the cases are manufactured, Lapua punches the holes and some cases will have a little crescent burr on the inside, and some do not.

Would there be any difference if I shot some with burrs left in, and those that had the burrs removed? The thought processes being that if I mixed and matched my cases, at long range say 1000yds would it cause vertical elevation problems.

To check the theory out I found 20 cases with burrs in them. I marked the headstamp with black felt tip where the burr was. I then chambered the round with the mark orientated at 12 o clock to maintain a degree of consistency. I also loaded up 20 rounds with no burrs on the inside (I did check both types with a bore scope down the inside of the case to ensure that burrs were present or not). I zeroed the rifle and then fired the cases with burrs first and then the ones without burrs.

At 100yds the burred cases had an impact point 1/2 an inch lower than the ones with no burrs. Weird isn't it. Has anyone else found this out? I know that in reality it will not make much difference to shooters one way or the other, but at long range in competition, it could cause you to loose valuable points!

 

 

 

[Gunsgobang88]

Hi,

 

Thanks for your post, for years I have wondered about the practical effects of flash hole de-burring. When assembling precision loads I always de-burred as a matter of course as it 'was the thing to do'. Did you notice any effect on group size or velocity consistency?

 

Alan

[brown dog]

I zeroed the rifle and then fired the cases with burrs first and then the ones without burrs.

At 100yds the burred cases had an impact point 1/2 an inch lower than the ones with no burrs. Weird isn't it. Has anyone else found this out?

 

At an intuitive level that strikes me as 'not right'. (That is, correlating the 0.5in change with burrs vs no burrs strikes me as 'not right )

 

I would strongly suspect something else caused the vertical change.

 

Was it overcast for the first group and the sun came out for the second group? With optics that'll raise your MPI by spot on 0.5MOA.

 

As a common-sense check. What MV change would be required for your load to rise 0.5 inches at 100? I reckon for the cause-effect you're postulating that the burrs would have to have affected the MV by 300 to 500fps to achieve an effect of the size you saw. Doesn't seem likely to me

[Dark Star]

In the testing I've done, vertical dispertion and MV are not as closely linked as might be assumed. Think about ladder tests. When you get the 'right' load, a significant variation in powder charge and MV won't make a lot of difference at the target.

Although I too am a bit sceptical that burrs in flash holes would make that much difference at the target, it's still conceivable that a change in the way the powder is ignited will change the POI, but I'm not sure it's as simple as just changing the MV. Maybe it has an affect on when or how long the peak pressure occurs and it's to do with harmonics?

 

Only one way to find out though - more testing!

[Laurie]

Lapua drills its flash-holes, unlike American brass where all the manufacturers use a punch. The result is that Lapua brass almost invariably has very round looking and apparently centrally placed holes as well as their being very consistent diameter-wise. Winchester / R-P etc brass with punched holes often has noticeably off-centre holes, different sizes and sometimes a visible elongation to the hole shape. Using standard deburr-flash-hole dia. uniforming tools often appears to have little or no effect on flash-hole diameter on Winchester etc brass as it's already well over the standard 0.080" diameter. Whether due to the forces involved in punching flash-holes or the way the manufacturers swage the primer pocket, US cases often have pockets with noticeably concave floors and anything but uniform depths. Lapua, RWS, and Norma brass don't suffer these problems, although I have got some Norma brass with too shallow pockets that need uniforming before a first loading or the primers stand slightly proud.

 

Punched hole edges are sometimes 'clean' on the case-interior, more usually have irregularly positioned spikes of varying lengths. Deburring is therefore essential with most batches of US brass. Doing some Hornady 6.5mm Creedmoor cases recently, I got a funny feel and sound from the deburr tool and two large chunks of metal came out, one a figure 8, the other a 3 with curling tail off one end, both around a quarter-inch long. That case just couldn't have performed the same as the rest in the box, assuming it woulkd have fired at all as the junk may have blocked the flash-hole. It wasn't very well anchored and would maybe have gone down the barrel after the bullet on the first firing. That's an extreme example, but illustrates why you need to deburr.

 

So fas ar Lapua goes, especially cases like 6BR and 6.5X47 Lapua that use the small primer / small flash-hole (1.5mm / 0.059"), my experience is that they're very uniform diameter. Others have said to me they've seen burrs on the inside, and although I've looked many times I've never found them myself. I only use a flash-hole uniforming tool on my Lapua brass, the Sinclair models that index from the primer pocket and don't worry about the exit end of the hole unless I can see anything odd. One problem with small flash-holes is that nearly all the hole uniformer / deburr tools are configured for the large hole size and will ream the small type out, a guaranteed and proven way of inceasing group size. K&M lists a PPC combined tool and that's the only one I've ever seen for such cases. Otherwise if you're worried about burrs on the BR, it's a case of use a pin-vice and micro drill bit slightly over 1.5mm dia.

 

One reason why you get deburr proponents for the BR and similar is that they believe a small bevel cut into the exit end of the flash-hole helps iginition through the so-called 'venturi' effect. Others say, don't do this at any price. I've never seen any evidence that either approach is better than the other. If you do cut a small venturi though, you'd need to be sure you're cutting cases consistently otherwise your 'uniforming' risks creating non-uniform cases!

 

My results with flash-hole diameter uniforming with the Sinclair tools is that any difference in cutting effort required between cases is minimal if it exists at all, all bar one box of .223 Rem match brass I loaded where the (standard size 0.080") holes definitely did show variations. Three cases took far more effort to uniform the holes than the other 97 in that one box and some brass chips fell out afterwards. In doing maybe one to to two thousand Lapua cases in recent years with both flash-hole diameters, that's the only time I've ever found a problem with them

[jcampbellsmith]

...... I zeroed the rifle and then fired the cases with burrs first and then the ones without burrs.

At 100yds the burred cases had an impact point 1/2 an inch lower than the ones with no burrs. ..

Unless you did a blind test and as has been pointed out elsewhere, took into account the weather, barrel fouling etc, I don't think the results prove anything. I think if you gave a good shot your rifle and randomly gave him rounds to fire, there wouldn't be any difference between the groups. Regards JCS