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Seating depth

26 Jan 2018
@ 11:13 am (GMT)

Clarence Roberts

I am looking for input on free throat for 208 grain 212 grain hornady eld bullets in a 300 R.U.M..


27 Jan 2018
@ 09:35 am (GMT)

Nathan Foster

Re: Seating depth
Hi Clarence, I have put the ins and outs of this in my Cartridges book which can be used in conjunction with the reloading book.

Max OAL's are right around 4". But you cannot seat to this length as the bullets will sit too far out of the case neck, plus, even with a Wyatt mag box, you are limited in internal length. Nevertheless, an M700 action and Wyatt box allow you to decrease the jump to some degree. ELD-M loads can be set at 3.740". This can make for a nice rig, even though barrel (or sweet spot) life is short, if practicing regularly with the rifle.

Note also that the max OAL of the .300 RUM is a meaningless temporary figure. The throat grows very quickly with wear. Consider 40 thou growth per hundred shots as being realistic.

One can see how and why the .300 Norma was created to overcome at least some of these issues. The rim diameter is annoying but the premise is sound.

28 Jan 2018
@ 05:06 am (GMT)

Clarence Roberts

Re: Seating depth
Do these bullets ( 208 &212 great.) Seem to like to be seated close to the rifling or do they function better with a longer jump
28 Jan 2018
@ 07:45 am (GMT)

Nathan Foster

Re: Seating depth
Hi Clarence, I do not normally answer mail or threads which show a lack of respect (hello / please / thank you). But you have asked a good question so I will answer it for the benefit of all.

The faster you drive these (and most other) projectiles, the closer they like to be seated to the lands. Also, if we can seat close to the lands, it gives a great deal of room for harmonic experimentation. The ELD-M is particularly sensitive to jacket upset during ignition, the ELD-X is very sensitive to lead angle and can require experimentaion both in close and out long if the magazine and cartridge design allow for this (the RUM does not).

The slower we go, the more we can get away with a measure of jump.

In the RUM's, you cannot seat close to the lands unless the throat is cut short which can open up new problems.

I have often thought that rather than cutting a magnum throat long or playing with bore riding throats (still suffer a sudden upset due to final sharp step down lead angle) a more harmonious approach would be to utilize a Brenneke style throat which is in essence an extremely long taper. I believe (but have no evidence) that this could help a RUM's a good deal.

Again, this subject is covered in my book series.

This is the last time I will answer this thread.
29 Jan 2018
@ 05:05 am (GMT)

Clarence Roberts

Re: Seating depth
thank you for the information i did not mean to sound rude.
29 Jan 2018
@ 06:52 am (GMT)

Nathan Foster

Re: Seating depth
You are most welcome Clarence. Perhaps one of the downsides of the internet, we risk losing some forms of interaction and communication.

To recap, based on my personal experience, the closer we get to and above 3000fps, the nicer it can be to have room in the magazine or within the cartridge design to experiment with bullet jump.

As we get down to 2700fps and below, a measure of freebore (.260, 7mm-08, .308) can be just fine, allowing some room for both power and accuracy development.

It is very common within the industry, to see factory ammo for most cartridges loaded to around 120 thou jump. This helps to ensure uniform pressures from rifle to rifle regardless of individual differences in chamber specs. But some cartridges are designed with extra freebore, as long as .400" which is designed for the sake of extra power development in possibly overbore cases. This can work, up to a point. But our ammunition must be concentric. The trouble is that an overbore cartridge may cause severe erosion, so the throat keeps growing.
29 Jan 2018
@ 08:56 am (GMT)

Thomas Kitchen

Re: Seating depth
Hi Clarence welcome to the forum.
thank you very much for that info Nathan.

i do find this stuff very interesting after do looking into throat designs with my reamers.
it appears the bore rider/stepped design was more about velocity then accuracy but it might go hand in hand.
the very slow taper throat design sound well worth while specially with more and more bullet makers releasing high bc projectiles (sierra and berger)
the concern would be getting the seating depth right as not to cramp powder and not to go past magazine length.

i went to help someone local with reloading last night, pretty much checked his run out, explained its best not to full length size specially belted magnum and recommended Nathan's reloading book.

he had has a Sako (yes i feel dirty even typing that word) in 300 win mag, checking magazine i was instantly angry that someone would build a win mag with such a short magazine under 86mm, checked action to see if single loading was an option but that's not much better so even if he wanted to seat long he may not be able to eject a live round.
there's not a lot of options really so will try and find a projectile that is forgiving with the jump and maybe running lower velocity then ideal.

Clarence a lot of info is in Nathan's reloading book, i find the way Nathan explains experimenting with jump and how to do it one of the most helpful things i have read.

29 Jan 2018
@ 10:04 am (GMT)

Nathan Foster

Re: Seating depth
Thanks for your kind words Thomas.

Clarence, what Thomas is speaking of is yet another problem in which the cartridge design is not overly long, but the gun maker (or more correctly the overseeing corporate) has applied severe cost cutting, resulting in a smaller action and therefore longer bullet jump with hand loads.

This is the downside of modern production and marketing. Cost cutting dressed up as "innovative technology". The companies give the customer less, then have the audacity to try and fob the customers into believing that it is a weight (or mass) saving for their benefit. Unfortunately, a large portion of hunters believe these marketing ploys. This fuels and drives the market further. Give me a range, a selection of rifles, a shooter of any experience level including new shooters (including women) and I can demonstrate this unequivocally in just a couple of hours.

To my new readers, understand that what you think you want versus what you need to get what you want may be two different things. You must learn to see through the current trends to understand this.


We are a small, family run business, based out of Taranaki, New Zealand, who specialize in cartridge research and testing, and rifle accurizing.