In spite of the economic downturn, London's specialist auction houses selling modern and antique guns have regularly posted staggering sales figures. Just five years ago, Holt's revealed that its December sale hit a record £1.7m. Among the hot sellers were a pair of Purdey shotguns, which sold for £80,000, and two guns made by Holland & Holland that went for £78,000.

"These are impressive and reassuring figures for prospective investors," explains auctioneer Nick Holt, adding: "Of course, not all guns command these toppy sums. Generally speaking you won't make a killing by investing in pistols, shotguns and rifles but their value will increase by around 3-5% each year. Furthermore, guns are classed as 'wasting assets' by HMRC, which means they are exempt from capital gains tax." This is all very positive news for buyers looking to broaden their portfolio.

So what's the key to finding that one-of-a-kind gem? Auctioneer Gavin Gardiner has managed Sotheby's sporting guns department in London for more than 30 years. "The three attributes you need to assess when considering a vintage gun are quality, condition and maker. Ideally you want to find a gun that has lived a sheltered life with very few miles on the clock. Sure, you can fix and replace knackered parts, but it is never as valuable as the original, untouched version. It is a false economy to buy a flashy gun in poor condition that needs a lot of restoration."

According to Gardiner, the best classic English shotgun makers for a safe investment are Purdey, Holland & Holland and Boss & Co. For bolt-action rifles also add newly revived London gunmaker John Rigby & Co to the list. "There is always a market for vintage guns from these recognisable makers, in particular Purdey. Everyone wants to own a Purdey."

Rigby's managing director, Marc Newton adds: "Rigby is the third oldest gunmaker in the world and has a cult following. Anything made before the Second World War will produce a good return as will the iconic Rising Bite double rifle and magnum-actioned .416s."

The three attributes you need to assess when considering a vintage gun are quality, condition and make. Ideally you want it to have led a sheltered life

The Antiques Roadshow's firearms expert Bill Harriman advises rookies to stick to traditional-looking guns. "Be aware of fashions, but stick to original, unaltered design features. For instance, if you're opting for a side-by-side shotgun, choose a double trigger. Don't be snooty about the humble boxlock – I'd opt for a showy boxlock over a decaying sidelock any day. Whatever your firearm, pick top grade wood and a well-known engraver. You want your gun to have global appeal."

Master engravers to watch out for include the doyen of gold game scenes Ken Hunt and his highly celebrated teacher, the late Harry Kell. "Any gun that boasts engraving work by these premier craftsmen will fetch a significant premium," reveals Gardiner.

Guns with a special provenance – those that were once owned by royalty or a champion shooter, for example – can accrue in value at an even greater rate. Gardiner explains: "Buyers like a story to go with their investment. I sold a pair of 16 bore Purdeys that were built for Edward VII for £131,000 in Geneva. Guns with this kind of provenance command eye-watering sums."

One way to keep abreast of the market is to request the Obsolete Calibres List from the Home Office. Holt points out that once a calibre has been struck off, it will shoot up in value. "Most recently, Swiss military rifles in 11.7 calibre were made obsolete. Overnight, this made them highly sought after. Like all investments, you'll need to do your groundwork."

One way of dipping your toe into gun investment is to consider joining the Gunroom Fund, run by Holt's. It was set up four years ago by Nick for members to jointly purchase and then sell modern and antique guns, with a view to gaining appreciation of initial capital contribution. "The fund is limited to just 10 members and is run over five years. There's around £130,000 in the pot at the moment. It has proved hugely popular, in particular with investors that are time and knowledge poor."

The CLA Game Fair is a one-stop shop for investing in top-grade guns. As the world's largest outdoor game fair, all the London gunmakers will lay out their very best showpiece items.

One last point to make is that your new acquisition can actually be used. "While it's not recommended to regularly take unique, antique or collectible guns to the firing range or grouse moor, a gun will not lose its value if it is well looked after," advises Gardiner, adding: "Guns are a surefire way of getting bang for your buck."

Gavin Gardiner's Fine Modern & Vintage Sporting Guns auction takes place on 29 August 2016 at the Gleneagle’s Hotel;