The short answer: print your own QR labels for things you name (tools, bins, shelves, gear) and reuse the manufacturer's barcode for things that already carry one (boxed stock, retail goods). A phone camera reads both, so you don't have to choose - you attach both identities to the same item and scan whichever is closest.
What each code is good at
A barcode (EAN/UPC) identifies a product: every box of the same cable ties carries the same code. Perfect for "how many of these do we have" - useless for "which drill is this one". A QR code you print yourself identifies a thing: THIS drill, THIS shelf, THIS bin. It scans reliably from any angle, at small sizes, on curved surfaces, even scuffed - which is why every tool-crib system converges on QR.
The setup that works in practice
Give every item a QR identity automatically (in ItemTally each item gets one at creation - IT-000214 style). Print labels in sheets on the office printer - Avery templates work - or on a Brother label roll for one-at-a-time. Stick QR labels on places too: a shelf label means "scan the shelf, see what lives there" and makes put-away idiot-proof, in the kindest sense.
For boxed consumables, don't print anything: scan the barcode that's already on the box and attach it to the item once. From then on, any crew member scanning that box lands on the right item, with the count one tap away.
Sizes, materials, durability
25mm square QR is comfortably scannable by any modern phone; go 38mm+ for anything read at arm's length or in bad light. Laminated or synthetic labels survive vans and workshops; paper is fine for office shelves. Put the human-readable name and code under the QR - when the label is half-worn, the text is the fallback. And avoid wrapping labels around tight curves; a flat tag zip-tied on beats a stretched sticker.
Do you need a scanner gun?
Usually no. A phone camera scans QR and barcodes, works offline, and is already in everyone's pocket. A USB gun earns its keep at a fixed station doing hundreds of scans an hour - it types into the search box like a keyboard, so it works with ItemTally without any setup - but it's an optimisation, not a requirement.
The mistake to avoid is philosophical, not technical: don't try to make one code type do both jobs. Products share barcodes; things deserve their own QR. Both, attached to the same register entry, and every scan lands in the same place: how labels work in ItemTally.