Throat erosion versus bullet jump?

[Monkey Spanker]

When I purchased my 25.06 about 5 years ago, I worked up a custom load which was 20 thou of an inch back from the lands. Lovely grouping - no problem! I got the max OAL by feeding a spent case with lightly fitted bullet head in so that the lands seated the bullet. However, I was curious to see whether this might have changed. To my surprise even with the head only just inside the case, it doesn't make it as far as the lands anymore!! The rifle is still very accurate. I can only assume this is due to throat erosion? If this is the case, the distance the bullet jumps must increase by a tiny amount every time the rifle is fired due to wear? My question is, should I be chasing this 20 thou" sweet spot or just keep my loads at the length it liked originally. As I can't make it 20thou back anymore I think I already know the answer, but I'm curious to know how barrel wear affects accuracy of developed loads as the bullet jump distance increases? Any thoughts or ideas welcomed.
Cheers, MS

 

[Brithunter]

Hey Monkey Spanker,

My one personal opinion is that a load of bull is spouted about bullet jump and it's influence on precision .................. wait a mo whilst I don me tin hat and flack jacket .

Now the thing that made me really think about this was firstly the Sporterised Swedish Mauser in 6.5x55 naturally and of course using 120 grain bullets it's impossible to get near the lands yet it shoots very well thank you. Then back in 2003 whilst doing final load development with my 7x57 BSA CF2 for a trip to the US after Whitetailed deer I found that although the Hornady 139 grain bullet I had acquired shot excellent groups it would not feed from one side of the magazine due to thier large flat metplat so I brought a box of Hornady 139 grain Spire Points which have a flat base but pointed profile. They didn't shoot the same as the flat points but eventually I found a good load but by that time I had used most of the box up.

On a vist to the gunshop for another boxI was horrified to find they didn't have any. They ordered some over night for me as time was begining to get short before departure and the new stock turned out to be 139 Gr BTSP trying to load them with the same recipe as the Spire Points gave a group of just under 3". Altering the powder charge and seating depth slightly didn't seem to help. The bullets were seated out to get close to the leade as we read how this is best for precision .

So in despair I went to the original powder charge but this time instead of seating out to get near the leade I seated the bullet to it's cannulure and the group haved in size . Ended up tweaking the seating depth out a little and with the cannulure out of the case mouth by about 1/16" the group tightened up to about 3/4" so I loaded up 60 of them and prepared the rest my stuff for the trip. By the time I got the load right I had about 12 days left until departure.

I also shoot 130 grain Hornady Spire Points in a .308 and a 30-30 bolt action neither of which can I get them near the leade and both are capable of shooting three shot groups that make an elongated ragged hole.

Your rifle you say is still giving good precision on target so I would say to you:-

If it ain't broke don't fix it

 

[Monkey Spanker]

Thanks mate, I don't intend to change anything, but I'm just trying to learn something. I know the 6.5x55 can be a problem when the ideal OAL won't then fit in a magazine!
You then have to keep shortening until you hit a second 'sweet spot' which is no doubt some second harmonic nodal point further back.
My point is, how can a rifle stay accurate when the bullet jump distance varies as the rifle wears? Maybe mine has worn back to a second sweet spot? That might account for any wild shots I might have done in between the two sweet spots!?
I'm sure someone will be along shortly with all the answers to my questions!
MS

 

[Brithunter]

Ahhhh Optimal Barrel Timing theory I don't pretend to understand it but have a friend in the US who believes it saves him time and componants in his testing and developing loads. However he is a scientist by profession and has brought a pressure trace system and combined with Quick load is well into the internal ballistics.

In fact he has been in touch with and is in correspondace with Quickloads developers as he found some quirks/errors if you like with it which they are addressing. He tried working out some OBT predicted loads for my 6mm Remington however it has not produeced the precison we were hopign for so that particular project is on the back burner until the summer now .

I do need to get a chrongraph to help with load development and checking and will endevour to get one before the summer if possible.

Throat or leade wear does not explain the preference of a longer bullet jump on the 7x57 as to date is has only fired around 525 rounds and it has fired some 197 rounds since that bit of load development so it only had 318 through it back then The rifle was new when I acquired it back in August or September 2001. Yes it was found tucked away in some old stock but the hang tag was missing only the thread was still attached to the trigger guard so I know how many rounds it has fired. It pays to have more than one to spread the wear out .

 

[Eyefor]

.... My question is, should I be chasing this 20 thou" sweet spot or just keep my loads at the length it liked originally.

..again, tin hat & flack jacket at the ready - but i think far too much emphasis is put on 5 / 10 / 15 / 20 / 25 thou off the lands seatings and a lot of reloaders are not getting the best out of their rifles because of this.

I bought a Ruger 77 MKII - but the VT (varmint target) model which grouped ok but not great. I then found on various sites / forums that guys were reloading various calibres at up to 240thou off the lands with spectacular results.

After several test loads I found two "sweet" spots for my rifle at 135 thou and 162 thou off the lands and it is amazingly accurate.

Further investigation on t'interweb led me to the barrel harmonics area of understanding - but I am a field shooter & not a paper puncher so the last 1/4" of grouping at 300 yards does not bother me.

Don't feel pressured into sticking with what doesn't work. Whack a few "loony" lengthed loads (100 / 125 / 150 / 200? off) and see what you get.

10p says you will be pleasantly surprised.

 

[finnbear270]

I'm in full agreement with the above, every rifle is different, as to seating into the lands / lead, I am of the opinion that this is inviting a pressure spike.

 

[nell]

I'm in full agreement with the above, every rifle is different, as to seating into the lands / lead, I am of the opinion that this is inviting a pressure spike.

Seating into the lands works fine,but as with all reloading MUST be worked up with care.

It is not recomended for hunting loads though as at some stage you are going to leave a bullet in the throat and have an action full of powder

Not a problem on the range as a chambered round will be fired...


Nell

 

[Monkey Spanker]

My head is in turmoil now and all that i thought i knew is in question.
My mate had a rifle where the lands ended up about 7 inches from the ogive due to wear but still grouped as tight as ever!
I am now thinking that you are not actually seating the bullet a optimum distance from the lands but the end of the barrel!
Surely that is how the harmonics work if wear seems to have no effect?
I'm not actually that bothered as long as my rifle stays accurate, but an engineering background makes me curious as to how these things work. I'm sure there are a multitude of factors in the overall equation but wonder which is the greatest?
MS

 

[ezzy6.5]

Perhaps i'm wrong. I would think that Harmonics have little or no bearing on the jump to the lands. the jump to the lands will effect how the Bullet enters the rifleing.
Harmonics affect the point at which the bullet leaves the rifle and consistent harmonics are needed for good accuracy.
as long as the bullet enters the bore straight it should work fine.
After a chat on here, i moved the 129gn bullet back off the lands in my 6.5 as there was little bullet in the case. accuracy improved as i moved it back (this might be to the fact it was in the csae further.

jmho.

Ezzy

 

[finnbear270]

Minimum bullet seating is usually accepted to be one calibre.

 

[Eyefor]

Minimum bullet seating is usually accepted to be one calibre.

....A lot of the Yanks (including Sierra & Barnes) suggest 75% or 80% of calibre diameter BUT (surely) you must consider if flat base ot boat tail 'cos on boat tail there will be less neck contact?

Never seen that mentioned.

Dunno