Barrels, bullets and benchrest

Don Gilchrist

When I became interested enough in centrefire benchrest shooting to give it a go, a Savage 110 with a heavy profile, 9″ twist barrel in 223 Remington was available at my local gunshop. With Savage’s reputation for out-of-the-box accuracy that became my introduction and it developed over a couple of years with a good scope, new stock and Basix SAV2 trigger.

At my club there were half a dozen guys shooting centrefire, mainly 6mm PPCs. Nothing shot better in my Savage than Berger 70gn VLD bullets pushed by 22.5gn of ADI Benchmark 2 and Lapua Match brass. If I shot well, I had 100yd aggregates in the mid to upper 0.4″ which was good enough to mix it with the PPCs, but at 200yds it was a different story. My slow-running 70gn projectiles were being knocked around far more than the fast-moving 68gn bullets from the PPCs – the small 223 Rem case meant my bullets were out in the breeze too long to group tightly at 200yds.

I needed more accuracy but thought the Savage 223 had hit its limit so I’d have to step up the power and didn’t feel the flexy receiver was worth re-barrelling. I knew about the fire-forming, neck-turning palaver inherent to 6PPC and the fact there are no factory offerings in 6PPC, so it was custom rifle ‘full-tilt’ or something else.

On the website, the cartridge showing most promise for my shooting was 6BR norma. Savage make a couple of offerings in 6BR and one of them, the LRPV model 12, was available which I bought with a 26″, 8″ twist barrel on a single-shot receiver. It came with a ‘target’ accu-trigger but the Basix drop-in replacement was better.

After a re-crown the gun shot well with Berger 80gn flat base tangent ogive, match bullets, Lapua match brass and 30.1gn of ADI 8208. On a good day I shot aggregates in the low 0.4s MoA. But there’s never enough accuracy once you’re hooked and the temptation of a custom barrel and chamber proved irresistible. So, the rig became a modified H-S Precision stock, factory receiver, Basix SAV2 trigger and 28″ PacNor, 3-groove, super-match barrel with 7″ twist.

Then my gunsmith asked: “What chamber do you want? Loose neck or tight? Long freebore or short? Whose brass will you shoot?” In researching the aforementioned I found a lot of 6BR shooters use Lapua factory brass, a 0.272″ no-turn neck and freebore of 0.073″ so I went with that. In addition to chambering and bedding the action I had Jerome Ziersch, gunsmith in SA’s Clare Valley, go over my Basix SA2 trigger. Jerome sees a few Savage F Class model 12s with Basix triggers and has a routine of trueing-up the bolt face and refining the sear/trigger interface and his efforts made a noticeable difference. What follows is a summary of the basic factors affecting rifle accuracy (see diagram for components of the chamber).

Before I fired a shot, I spent a day with the projectiles on my shortlist, a Stoney Point gauge and bullet comparator to find where the rifling started, set up my seating die and made sure the freebore would match my range of projectiles. Freebore is the little bit of un-rifled bore just after the neck shoulder and before the tapered start of the rifling.

Understanding freebore means looking at bullet shapes and how they engage the rifling when fired. The pointy part of the bullet is the ogive and there are two main shapes. With a conventional ‘tangent’ ogive, the junction of the ogive to the body of the bullet is smooth and that’s where the bullet engages the rifling.

Low-drag, ballistically more efficient bullets have a longer, pointier nose with a ‘secant’ ogive. A secant/body junction is more abrupt which is why secant bullets are far more sensitive to seating depth and bullet makers usually put a taper at the back of the bullet called a ‘boat-tail’ to increase flight efficiency. The result of this quest for ballistic virtue is a long, pointy bullet with a boat-tail but one with a relatively short body, the only part of the bullet which actually engages with the case neck, bore and rifling.

If the freebore is too long, the body of the bullet can emerge from the neck before the ogive has caught the rifling and as the bullet rattles down the freebore it may not be centred along the bore when it fully engages the rifling. If the freebore is too short and a heavy, tangent ogive bullet has a boat-tail, the entire body and boat-tail can be in the neck and projecting back into the case, compressing the powder load with risk of excessive pressure.

In 6BR chambers there’s usually no problem with heavier bullets with boat-tails as the extra weight is carried in a longer bullet body. In lighter secant bullets it becomes a factor as the body has to be quite short to carry both secant ogive and boat-tail. The freebore treads a fine line which can limit the choices shooters have, particularly VLD or boat-tail bullets in the 90-95gr range so my bullet choices will be flat-based up to 90gr or boat-tails of 105gr.

At the lands, jump or jam?

If the bullet is seated so the junction of ogive and body just touches the start of the rifling when chambered, that seating depth is ‘at the lands’. With bullet seated 0.010^ further into the case it gets a run-up before making contact with the rifling and is ‘jumped’ 0.010″. If the bullet is seated 0.010″ longer than ‘at the lands’ it’s already in solid contact with the rifling by the act of chambering the round. That bullet is ‘jammed’ 0.010″.

With Berger 80gn FB projectiles I found some serious encouragement around 30gn of ADI 8208. I was firing only three shots, paying little attention to the wind and grouping in the 0.2s”. As you master break-in, fire-forming and load development you get an idea of what a good load might be. For what follows it makes sense to assume we’re starting with a new gun or barrel or a second-hand gun you’re unfamiliar with. My sequence runs something like this:

1: Start with new brass all the same batch. Lapua costs plenty but if treated gently 200 match grade cases last for years, 200 cases being several cycles of competition shooting if you maintain cleaning and case preparation. I’ve reloaded Lapua 223 brass dozens of times and never had a split neck or loose primer pocket.

2: Select the bullets you’ll use with care. I shoot 100 and 200yds so prefer FB bullets in the 80-90gr range. Choose bullets you can buy reliably – Bergers work for me but availability’s an issue. Find the base-to-ogive measurement that reliably places your bullets and record it so you can set any amount of or your gun likes with your seating die. I use Redding micrometer competition dies and without equivalent kit you won’t be in the hunt. Every bullet is different so measurements must be done for each weight, type or manufacturer. I seat bullets ‘at the lands’ to begin with which has never given excessive pressure signs and has proven to be close for my best loads.

3: Choose conservative starting loads. Initially you’re only fire-forming so top accuracy can’t be expected (you can fine-tune loads, bullets and seating depth to tighten groups later). Start 10 per cent below maximum recommended loads and work up 0.5gn increments.

4: Fire three-shot groups of each load. If after three shots the group looks dodgy it’s not going to improve after five.

5: Check for signs of pressure (I use the primers as a guide). There are many YouTube clips on what to look for and always using the same primer helps.

The vital components are the locked bolt face and chamber, bullet and barrel and the first critical factor is all of these must be perfectly co-linear and symmetrical, with the locked bolt face exactly at right angles to this axis of symmetry. Barrel and chamber are big contributors to accuracy and little can be done about that without major gun surgery.

Next of the major factors you can control are the brass, bullet, powder load and seating depth. The 6BR is such a forgiving cartridge there’s a saying among top US shooters: “Use a standard Lapua case, CCI 450 primer, seat a Berger 105 at the lands and push it with 30gr of Varget (AR 2208). If the gun won’t shoot that it’s not the load’s fault.”

The hardware of the action, chamber and barrel accounts for about 60 per cent of a gun’s potential accuracy, case, powder and bullet can add another 20 per cent, competition dies and seating depth maybe 10 per cent and so on as you strive for that un-attainable 0.0^ group.

After three fire-forming sessions breaking in the new barrel and load development, there will be enough brass and load data for competition. I load 50 rounds for the comp and up to 10 extras to get the gun going, check fall of shot at the change of distance, plus a few spares. I may also load up to 15 shots of testing on the same day.

I live in the tropics where heat and humidity are high and variable and learned the hard way that all competition shots and extras should be loaded at the same session. If you’re jamming your bullets it’s a good idea to test a round to make sure it chambers properly before heading to the range.

Handloading for benchrest competition is about uniformity, your gun’s chamber sets the case form and the barrel is what it is. Everything else is about controlling the variables that affect the projectile leaving the case neck and engaging the rifling.

Benchrest shooting is a challenging, disciplined and cerebral sport. You’ll never shoot a perfect competition but you can do better than you did last week or maybe better than anyone else on the day. You can’t control the weather but you can learn to assess the best wind conditions on offer.

Read and believe the SSAA’s Brendan Atkinson’s Ten Tips for Benchrest Shooters especially the first and last: Proper Preparation Prevents Poor Performance; and Never, ever give up. If you don’t want to study old targets or look forward to hours at the loading bench cleaning, preparing and loading your ammo then centrefire benchrest isn’t for you. Luck has nothing to do with shooting – the last shot went exactly where you put it. Every shot has a lesson for you, learn it and apply it to your next shot.

All News