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Posted: 9/3/2008 8:30:16 PM EDT
A 162 gr A-Max 7mm in a 1:10 twist barrel in 7mm RUM. It is shooting all over...not accurate at all. I think the book calls for a 1:9. Should 1 make that much of a difference?

Plus... My new 7mm rum has free bore out the ass.. if I load to the lands the bullet is out of the case, by far. Can't do it.  Can I have the barrel turned and reamed to get rid of the free bore or does 7mm rum need that much free bore?

I love this cartridge. It is so flat shooting and super fast. Dead on at 100 yrds up to 300 yrds, same point of aim.
Link Posted: 9/3/2008 8:42:35 PM EDT
A long freebore lowers pressures.  If factory ammo is developed for a long freebore, you will blow primers if you reduce the freebore length. Short freebores are not all they are cracked-up to be. Unless you have a super tight chamber body that aligns the bullet with the bore by virtue of case geometry, you need a freebore at least half as long as the bullet shank to straighten out the bullet before it hits the rifling. Think about it:

You have a chamber with .005" of clearance at the base and your bullet is jammed into the rifling. Just how straight will that bullet enter the bore if the case is cocked by .005" against one side of the chamber under extractor pressure?

Benchrest chambers do not have this problem because they have maybe .0005" of clearance and no runout. The case itself aligns the bullet with the bore. That is why you can get away with a super short freebore. In fact, some chamberings, like the .30 BR, do not have a freebore at all.

In my opinion, if you want to use factory ammo, a long freebore is just fine so long as it is concentric with the bore centerline and tight enough in diameter so that the bullet cannot wobble, i.e. no more than .0005" over bullet diameter.
Link Posted: 9/3/2008 9:26:55 PM EDT
Like above, that freebore is necessary for that cartridge as it is far more capacity than normal.

What were the bullet holes looking like?  How about the velocity?  Standard deviation?
Link Posted: 9/3/2008 9:39:48 PM EDT
[Last Edit: 9/3/2008 9:41:28 PM EDT by sailhertoo]
93.3 gr retumbo seems to be most accurate. Shooting about 3250 fps. This is with 160 gr nosler accupoint pills. 85 degrees Fahrenheit, 2100 ft. elv. Standard dev. is fluctuating from 11 to 18 in that load.

So should I not have the work done to get rid of the free bore? does it need that much freebore in that cartage?
Link Posted: 9/3/2008 9:46:52 PM EDT

Originally Posted By 918v:
A long freebore lowers pressures.  If factory ammo is developed for a long freebore, you will blow primers if you reduce the freebore length. Short freebores are not all they are cracked-up to be. Unless you have a super tight chamber body that aligns the bullet with the bore by virtue of case geometry, you need a freebore at least half as long as the bullet shank to straighten out the bullet before it hits the rifling. Think about it:

You have a chamber with .005" of clearance at the base and your bullet is jammed into the rifling. Just how straight will that bullet enter the bore if the case is cocked by .005" against one side of the chamber under extractor pressure?

Bench rest chambers do not have this problem because they have maybe .0005" of clearance and no runout. The case itself aligns the bullet with the bore. That is why you can get away with a super short freebore. In fact, some chamberings, like the .30 BR, do not have a freebore at all.

In my opinion, if you want to use factory ammo, a long freebore is just fine so long as it is concentric with the bore centerline and tight enough in diameter so that the bullet cannot wobble, i.e. no more than .0005" over bullet diameter.


You make a lot of since to me. But , think
'bout your last paragraph, is that not contradictory?
Link Posted: 9/3/2008 10:21:33 PM EDT
Freebore diameter should not be over the bullet diameter by a half-thousandth of an inch.  The reason is to get volume before the engraving force causes pressure to spike.  That pressure spike causes the entire charge to ignite and this will cause pressure to rise out of control if the engraving force increases.

Basically, the bullet travels a short distance before encountering any significant resistance.
Link Posted: 9/3/2008 10:55:14 PM EDT

Originally Posted By Keith_J:
Freebore diameter should not be over the bullet diameter by a half-thousandth of an inch.  The reason is to get volume before the engraving force causes pressure to spike.  That pressure spike causes the entire charge to ignite and this will cause pressure to rise out of control if the engraving force increases.

Basically, the bullet travels a short distance before encountering any significant resistance.


If I'm reading you right, your talking bout freebore dia. , not "freebore", Lenght before the bullit hits the rifleing?
Link Posted: 9/4/2008 12:03:20 AM EDT

Originally Posted By sailhertoo:

Originally Posted By Keith_J:
Freebore diameter should not be over the bullet diameter by a half-thousandth of an inch.  The reason is to get volume before the engraving force causes pressure to spike.  That pressure spike causes the entire charge to ignite and this will cause pressure to rise out of control if the engraving force increases.

Basically, the bullet travels a short distance before encountering any significant resistance.


If I'm reading you right, your talking bout freebore dia. , not "freebore", Lenght before the bullit hits the rifleing?


Yes, with MOST bullets, an increase in freebore does very little harm to accuracy as long as the bullet fits into the freebore and the freebore is concentric with the groove diameter.  This was harder to do with a pilotless reamer.  Bear in mind, such a reamer has 1-2 thousandths of an inch of clearance with the bore diameter.  But when the reamer is piloted, the throat can be made concentric with the groove diameter if the rifling was done uniformly.

A chamber cast with Cerrosafe will show if there are problems here.  Include all the freebore plus the leade.  Don't start hacking unless you know what the problem is...

I bet that rifle shoots fine with 150 grain bullets.  With a long freebore, about the only bullets that MIGHT have problems are the VLDs.  But those would be too fragile for hunting anyway.
Link Posted: 9/4/2008 12:19:18 AM EDT
[Last Edit: 9/4/2008 12:22:37 AM EDT by sailhertoo]
Thanks,

Man, you know what your talking 'bout .

I appreciate your in put.

Edit: don't mean to sound stupid but I guess I am, Where are you from, "GEO"?
Link Posted: 9/4/2008 1:11:41 AM EDT

Originally Posted By sailhertoo:
Thanks,

Man, you know what your talking 'bout .

I appreciate your in put.

Edit: don't mean to sound stupid but I guess I am, Where are you from, "GEO"?


That is the New Georgia, not the one above your state

I'm a Texan.  But I support Georgia and their continued soverign independence.

I'm no gunsmith by trade but I have chambered and rebarreled a few rifles. My .308, built on a long action, has a long freebore of 0.3085 with an identical groove diameter.  I can load to an OAL of 3" without any contact.  This gives me additional powder space which lets me get 3050 FPS with a 150 grain Sierra.  Even though it is a 1:10" twist, that load prints groups that are a bit under 2.25" at 300 yards, on calm days without mirage.  It does even better with 168 grain Matchkings.  If I had a proper scope in the 30 power range and a benchrest stock, it would easily do 1/2 MOA.  But it is a fun rifle and I know where the bullets will impact.  

Everyone should have an accurate rifle.  And know how to use it.  
Link Posted: 9/4/2008 1:55:25 AM EDT

Originally Posted By Keith_J:

Originally Posted By sailhertoo:
Thanks,

Man, you know what your talking 'bout .

I appreciate your in put.

Edit: don't mean to sound stupid but I guess I am, Where are you from, "GEO"?


That is the New Georgia, not the one above your state

I'm a Texan.  But I support Georgia and their continued sovereign independence.



Are you talking 'bout the one Russia just invaded? Are you there now?

If so, God bless.
Link Posted: 9/4/2008 1:59:07 AM EDT
Before I bought my gun I did alot of research, the 7mm rum hase the best ballistics of them all.... don;t understand why more people doh't lean that way.
Link Posted: 9/4/2008 3:58:36 AM EDT
Link Posted: 9/4/2008 7:47:53 AM EDT

Originally Posted By Keith_J:
Freebore diameter should not be over the bullet diameter by a half-thousandth of an inch.  The reason is to get volume before the engraving force causes pressure to spike.  That pressure spike causes the entire charge to ignite and this will cause pressure to rise out of control if the engraving force increases.

Basically, the bullet travels a short distance before encountering any significant resistance.


Yes it should. If it is the same as bullet diameter, the cartirige likely won't chamber because most rounds have some runout. If you consult a reamer manufacturer, you'll see that most match chambers have a freebore diameter only .0005" larher that the bullet diameter, some as little as .0002".
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